Used to describe the unit of rhythm in languages displaying isochrony, i.e. where the stressed syllables fall at approximately regular intervals throughout an utterance. It is an extension of the term used in traditional studies of metrical verse structure, where the many regular patterns of stressed/unstressed syllables were given a detailed classification. In a more general phonological sense, the notion is applied to any utterance in a stress-timed language, not just verse. [Crystal 1985: 123-124]