A woman (irregular plural: women) is a female human. The term woman is usually reserved for an adult, with the term girl being the usual term for a female child or adolescent. However, the term woman is also sometimes used to identify a female human, regardless of age, as in phrases such as "Women's rights".
The English term "Man" (from Proto-Germanic mannaz "man, person") and words derived therefrom can designate any or even all of the human race regardless of their sex or age. This is the old usage of "Man" in English. It derives from Proto-Indo-European *mánu- 'man, human', cognate to Sanskrit manu, Old Church Slavonic mǫžĭ, 'man', 'husband'.
In Older English the words wer and wyf (also wæpman and wifman) were used to refer to "a man" and "a woman" respectively, and "Man" was gender-neutral. In Middle English man displaced wer as term for "male human", whilst wifman (which eventually evolved into woman) was retained for "female human". ("Wif" also evolved into the word "wife".) "Man" carries this old sense of "Human" however, resulting in an asymmetry criticized as sexist. (See also womyn.)
A very common Indo-European root for woman, *gwen-, is the source of English queen (Old English cwēn primarily meant woman, highborn or not; this is still the case in Danish, with the modern spelling kvinde), as well as gynaecology (from Greek gynē), banshee fairy woman (from Irish bean woman, sí fairy) and zenana (from Persian zan). The Latin fēmina, whence female, is likely from the root in fellāre (to suck), referring to breastfeeding. 
The symbol for the planet Venus is the sign also used in biology for the female sex. It is a stylized representation of the goddess Venus's hand mirror or an abstract symbol for the goddess: a circle with a small equilateral cross underneath (Unicode: ♀). The Venus symbol also represented femininity, and in ancient alchemy stood for copper. Alchemists constructed the symbol from a circle (representing spirit) above an equilateral cross (representing matter).
Womanhood is the period in a female's life after she has transitioned from girlhood, at least physically, having passed the age of menarche. Many cultures have rites of passage to symbolize a woman's coming of age, such as confirmation in some branches of Christianity, bat mitzvah in Judaism, or even just the custom of a special celebration for a certain birthday (generally between 12 and 21).
Currently in the English language there is no commonly-used word for a woman who has passed menopause, although historically a woman in the third part of her life was known as a crone, which was originally not a pejorative term. The three ages of woman were historically known as "maiden, matron, and crone" and are sometimes quoted as "maiden, mother and crone". This could perhaps be rendered in modern English as "little girl", "woman of reproductive age" and "older lady".
The word woman can be used generally, to mean any female human, or specifically, to mean an adult female human as contrasted with girl. The word girl originally meant "young person of either sex" in English; it was only around the beginning of the 16th century that it came to mean specifically a female child. Nowadays girl sometimes is used colloquially to refer to a young or unmarried woman. During the early 1970s feminists challenged such use, and use of the word to refer to a fully grown woman may cause offence. In particular, previously common terms such as office girl are no longer used.
Conversely, in certain cultures which link family honor with female virginity, the word girl is still used to refer to a never-married woman; in this sense it is used in a fashion roughly analogous to the obsolete English maid or maiden. Referring to an unmarried female as a woman may, in such a culture, imply that she is sexually experienced, which would be an insult to her family.
In some settings, the use of girl to refer to an adult female is a common practice (such as girls' night out), even among some elderly women. In this sense, girl may be considered to be the analogue to the British word bloke for a man, although it again fails to meet the parallel status as an adult. Gal aside, some feminists cite this lack of an informal yet respectful term for women as misogynistic; they regard non-parallel usages, such as men and girls, as sexist.
There are various words used to refer to the quality of being a woman. The term "womanhood" merely means the state of being a woman, having passed the menarche; "femininity" is used to refer to a set of supposedly typical female qualities associated with a certain attitude to gender roles; "womanliness" is like "femininity", but is usually associated with a different view of gender roles; "femaleness" is a general term, but is often used as shorthand for "human femaleness"; "distaff" is an archaic adjective derived from women's conventional role as a spinner, now used only as a deliberate archaism; "muliebrity" is a "neologism" (derived from the Latin) meant to provide a female counterpart of "virility", but used very loosely, sometimes to mean merely "womanhood", sometimes "femininity", and sometimes even as a collective term for women.
In terms of biology, the female sex organs are involved in the reproductive system, whereas the secondary sex characteristics are involved in nurturing children or, in some cultures, attracting a mate. The ovaries, in addition to their regulatory function producing hormones, produce female gametes called eggs which, when fertilized by male gametes (sperm), form new genetic individuals. The uterus is an organ with tissue to protect and nurture the developing fetus and muscle to expel it when giving birth. The vagina is used in copulation and birthing (although the word vagina is often colloquially and incorrectly used for the vulva or external female genitalia, which also includes the labia, the clitoris, and the female urethra). The breast evolved from the sweat gland to produce milk, a nutritious secretion that is the most distinctive characteristic of mammals, along with live birth. In mature women, the breast is generally more prominent than in most other mammals; this prominence, not necessary for milk production, is probably at least partially the result of sexual selection. (For other ways in which men commonly differ physically from women, see Man.)
An imbalance of maternal hormonal levels and some chemicals (or drugs) may alter the secondary sexual characteristics of fetuses. Most women have the karyotype 46,XX, but around one in a thousand will be 47,XXX, and one in 2500 will be 45,X. This contrasts with the typical male karotype of 46,XY; thus, the X and Y chromosomes are known as female and male, respectively. Unlike the Y chromosome, the X can come from either the mother or the father, thus genetic studies which focus on the female line use mitochondrial DNA.
Biological factors are not sufficient determinants of whether a person considers themselves a woman or is considered a woman. Intersex individuals, who have mixed physical and/or genetic features, may use other criteria in making a clear determination. There are also transgender or transsexual women, who were born or physically assigned as male at birth, but identify as women; there are varying social, legal, and individual definitions with regard to these issues. (See transwoman.)
Although fewer females than males are born (the ratio is around 1:1.05), due to a longer life expectancy there are only 81 men aged 60 or over for every 100 women of the same age, and among the oldest populations, there are only 53 men for every 100 women. Women typically have a longer life expectancy than men. This is due to a combination of factors: genetics (redundant and varied genes present on sex chromosomes in women); sociology (such as not being expected in most countries to perform military service); health-impacting choices (such as suicide or the use of cigarettes, and alcohol); the presence of the female hormone estrogen, which has a cardioprotective effect in premenopausal women; and the effect of high levels of androgens in men. Out of the total human population, there are 101.3 men for every 100 women (source: 2001 World Almanac).
Most women go through menarche and are then able to become pregnant and bear children. This generally requires internal fertilization of her eggs with the sperm of a man through sexual intercourse, though artificial insemination or the surgical implantation of an existing embryo is also possible (see reproductive technology). The study of female reproduction and reproductive organs is called gynaecology. Women generally reach menopause in their late 40s or early 50s, at which point their ovaries cease producing estrogen and they can no longer become pregnant.
To a large extent, women suffer from the same illnesses as men. However, there are some diseases that primarily affect women, such as lupus. Also, there are some sex-related illnesses that are found more frequently or exclusively in women, e.g., breast cancer, cervical cancer, or ovarian cancer. Women and men may have different symptoms of an illness and may also respond differently to medical treatment. This area of medical research is studied by gender-based medicine.
During early fetal development, embryos of both sexes appear gender-neutral; the release of hormones is what changes physical appearance male or female. As in other cases without two sexes, such as species that reproduce asexually, the gender-neutral appearance is closer to female than to male.
See main article: Gender role. In many prehistoric cultures, women assumed a particular cultural role. In hunter-gatherer societies, women were generally the gatherers of plant foods, small animal foods, fish, and learned to use dairy products, while men hunted meat from large animals.
In more recent history, the gender roles of women have changed greatly. Traditionally, middle-class women were typically involved in domestic tasks emphasizing child care. For poorer women, especially working class women, this often remained an ideal, as economic necessity compelled them to seek employment outside the home. The occupations that were available to them were, however, lower in pay than those available to men.
As changes in the labor market for women came about, availability of employment changed from only "dirty", long houred factory jobs to "cleaner", more respectable office jobs where more education was demanded, women's participation in the U.S. labor force rose from 6% in 1900 to 23% in 1923. These shifts in the labor force led to changes in the attitudes of women at work, allowing for the revolution which resulted in women becoming career and education oriented.
Movements advocate equality of opportunity for both sexes, and equal rights irrespective of gender. Through a combination of economic changes and the efforts of the feminist movement, in recent decades women in most societies now have access to careers beyond the traditional homemaker.
The gender gap in OECD countries has been reduced over the last 30 years. Younger women today are far more likely to have completed a tertiary qualification: in 19 of the 30 OECD countries, more than twice as many women aged 25 to 34 have completed tertiary education than women aged 55 to 64 do. In 21 of 27 OECD countries with comparable data, the number of women graduating from university-level programmes is equal to or exceeds that of men. 15-year-old girls tend to show much higher expectations for their careers than boys of the same age.
While women account for more than half of university graduates in several OECD countries, they receive only 30% of tertiary degrees granted in science and engineering fields, and women account for only 25% to 35% of researchers in most OECD countries.