What is an Anacrusis in music?

In music, an anacrusis (Ancient Greek: ἀνάκρουσις, romanized: anákrousis, lit. 'up-beat') is a note or notes which occur just before the regular stressed beat of a bar, and which are then followed by a stressed beat on the first beat. The anacrusis, if present, is separated by a barline from the first beat of the bar, and is in a separate time signature from the main part of the bar if notated at all.

In Western music, the anacrusis is an important element of rhythm, and is used extensively in a wide range of genres, from classical music to pop and rock.