What is "isochrony"

n. The state of being isochronous.

Isochrony is the postulated rhythmic division of time into equal portions by a language. Rhythm is an aspect of prosody, others being intonation, stress and tempo of speech.

Three alternative ways in which a language can divide time are postulated:

1. The duration of every syllable is equal (syllable-timed);
2. The duration of every mora is equal (mora-timed).
3. The interval between two stressed syllables is equal (stress-timed);

The idea as such was first expressed by Kenneth L. Pike in 1945, though the concept of language naturally occurring in chronologically and rhythmically equal measures is found at least as early as 1775 (in Prosodia Rationalis). This has implications for language typology: D. Abercrombie claimed "As far as is known, every language in the world is spoken with one kind of rhythm or with the other ... French, Telugu and Yoruba ... are syllable-timed languages, ... English, Russian and Arabic ... are stress-timed languages'. While many linguists find the idea of different rhythm types appealing, empirical studies have not been able to find acoustic correlates of the postulated types, calling into question the validity of these types. However, when viewed as a matter of degree, relative differences in the variability of syllable duration across languages have been found.