Definify.com

Webster 1913 Edition

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Noun.

[Gr. [GREEK], fr. [GREEK] to push up or back; [GREEK] + [GREEK] to strike.]

(Pros.)

A prefix of one or two unaccented syllables to a verse properly beginning with an accented syllable.

Beginning of BWV 736, with an anacrusi shown in red.

anacrusis ‎(plural anacruses)

(prosody) An unstressed syllable at the start of a verse.
(music) An unstressed note or notes before the first strong beat (or downbeat) of a phrase.
- 1989, Anthony Burgess, Any Old Iron:
  Then Etheridge poised his baton, jerked an upbeat, and made the violinists speak the low G and A of their anacrusis.

(prosody) an unstressed syllable at the start of a verse

(music) an unstressed note (or notes) before the first strong beat of a phrase

anacrusis