In linguistics, stress is the relative emphasis that may be given to certain syllables in a word. The term is also used for similar patterns of phonetic prominence inside syllables. The word accent is sometimes also used with this sense.
The ways stress manifests itself in the speech stream are highly language dependent. In some languages, stressed syllables have a higher or lower pitch than non-stressed syllables so-called pitch accent (or musical accent). In other languages, they may bear either higher or lower pitch than surrounding syllables (a pitch excursion), depending on the sentence type. There are also dynamic accent (loudness), qualitative accent (full vowels), and quantitative accent (length, known in music theory as agogic accent). Stress may be characterized by more than one of these characteristics. Further, stress may be realized to varying degrees on different words in a sentence; sometimes the difference between the acoustic signals of stressed and unstressed syllables may be minimal.
In English, stress is most dramatically realized on focussed or accented words. For instance, consider the dialogue
"Is it brunch tomorrow?"
"No, it's dinner tomorrow."
In it, the stress-related acoustic differences between the syllables of "tomorrow" would be small compared to the differences between the syllables of "dinner", the emphasized word. In these emphasized words, stressed syllables such as "din" in "dinner" are louder and longer.   They may also have a different fundamental frequency, or other properties. Unstressed syllables typically have a vowel, which is closer to a neutral position (the schwa), while stressed vowels are more fully realized. In contrast, stressed and unstressed vowels in Spanish share the same quality—unlike English, the language has no reduced vowels.
(Much literature emphasizes the importance of pitch changes and pitch motions on stressed syllables, but experimental support for this idea is weak. Nevertheless, most experiments do not directly address the pitch of speech, which is a subjective perceived quantity. Experiments typically measure the speech fundamental frequency, which is objectivelymeasurable, and strongly correlated with pitch, but not quite the same thing.)
The possibilities for stress in tone languages is an area of ongoing research, but stress-like patterns have been observed in Mandarin Chinese. They are realized as alternations between syllables where the tones are carefully realized with a relatively large swing in fundamental frequency, and syllables where they are realized "sloppily" with typically a small swing.
Stressed syllables are often perceived as being more forceful than non-stressed syllables. Research has shown, however, that although dynamic stress is accompanied by greater respiratory force, it does not mean a more forceful articulation in the vocal tract.
Some languages have fixed stress. That is, stress is placed always on a given syllable, as in Finnish and Hungarian (stress always on the first syllable) or Quechua and Polish (stress always on the penult: one syllable before the last) or on third syllable counting backwards (the antepenult), as in Macedonian (see: Stress in Macedonian language). Other languages have stress placed on different syllables but in a predictable way, as in Classical Arabic and Latin (where stress is conditioned by the structure of the penultimate syllable). They are said to have a regular stress rule.
French words are sometimes said to be stressed on the final syllable, but actually French has no word stress at all. Rather, it has a prosody whereby the final or next-to-final syllable of a string of words is stressed. This string may be equivalent to a clause or a phrase. However, when a word is said alone, it receives the full prosody and therefore the stress as well.
There are also languages like English, Italian,Russian and Spanish, where stress is (at least partly) unpredictable. Rather, it is lexical: it comes as part of the word and must be memorized, although orthography can make stress unambiguous for a reader, as is the case in Spanish and Portuguese. In such languages, otherwise homophonous words may differ only by the position of the stress (e.g. incite and insight in English), and therefore it is possible to use stress as a grammatical device.
English does this to some extent with noun-verb pairs such as a récord vs. to recórd, where the verb is stressed on the last syllable and the related noun is stressed on the first; record also hyphenates differently: a réc-ord vs. to re-córd. The German language does this with certain prefixes for example úm-schrei-ben (to rewrite) vs. um-schréi-ben (to paraphrase, outline) and in Russian this phenomenon often occurs with different cases of certain nouns (земли́/zemli (genitive case of the Earth, land or soil) and зе́мли (soils or lands plural form)).
It is common for dialects to differ in their stress placement for some words. For example, in British English, the word "laboratory" is pronounced with primary stress on the second syllable, while American English stresses the first.
See main article: Secondary stress. 'Primary' and 'secondary' stress are distinguished in some languages. English is commonly believed to have two levels of stress, as in the words cóunterfòil and còunterintélligence, and in some treatments has even been described as having four levels, primary, secondary, tertiary, and quaternary, but these treatments often disagree with each other. It is possible to describe English with only one degree of stress, as long as unstressed syllables may occur without vowel reduction.
In many languages, such as Russian and English, vowel reduction may occur when a vowel changes from a stressed to an unstressed position. In English, many unstressed vowels reduce to schwa-like vowels, though the details vary with dialect. Other languages, such as Finnish, have no unstressed vowel reduction.
It is common for stressed and unstressed syllables to behave differently as a language evolves. For example, in the Romance languages, the original Latin short vowels and have generally become diphthongs when stressed. Since stress takes part in verb conjugation, this has produced verbs with vowel alternation in the Romance languages. For example, the Spanish verb volver has the form volví in the past but vuelvo in the present (see Spanish irregular verbs). Italian shows the same phenomenon, but with alternating with instead. This behaviour is not confined to verbs; for example, Spanish viento "wind", from Latin ventum.
English is a stress-timed language; that is, stressed syllables appear at a roughly constant rate, and non-stressed syllables are shortened to accommodate this. Other languages have syllable timing (e.g. Spanish) or mora timing (e.g. Japanese), where syllables or morae are spoken at a roughly constant rate regardless of stress.
Different systems exist for indicating syllabification and stress.