Since stress can be realised through a wide range of phonetic properties, such as loudness, vowel length, and pitch, which are also used for other linguistic functions, it is difficult to define stress solely phonetically.
The stress placed on syllables within words is called word stress or lexical stress. Some languages have fixed stress, meaning that the stress on virtually any multisyllable word falls on a particular syllable, such as the penultimate (e.e. Polish) or the first. Other languages, like English and Russian, have variable stress, where the position of stress in a word is not predictable in that way. Sometimes more than one level of stress, such as primary stress and secondary stress, may be identified. However, some languages, such as French and Mandarin, are sometimes analyzed as lacking lexical stress entirely.
The stress placed on words within sentences is called sentence stress or prosodic stress. This is one of the three components of prosody, along with rhythm and intonation. It includes phrasal stress (the default emphasis of certain words within phrases or clauses), and contrastive stress (used to highlight an item − a word, or occasionally just part of a word − that is given particular focus).
There are various ways in which stress manifests itself in the speech stream, and these depend to some extent on which language is being spoken. Stressed syllables are often louder than non-stressed syllables, and may have a higher or lower pitch. They may also sometimes be pronounced longer. There are sometimes differences in place or manner of articulation – in particular, vowels in unstressed syllables may have a more central (or "neutral") articulation, while those in stressed syllables have a more peripheral articulation. 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 are minimal.
These particular distinguishing features of stress, or types of prominence in which particular features are dominant, are sometimes referred to as particular types of accent – dynamic accent in the case of loudness, pitch accent in the case of pitch (although this term usually has more specialized meanings), quantitative accent in the case of length, and qualitative accent in the case of differences in articulation. These can be compared to the various types of accent in music theory. In some contexts, the term stress or stress accent is used to mean specifically dynamic accent (or as an antonym to pitch accent in its various meanings).