An anacrucis, known more informally as a pickup (see pickup notes and measures), is an unstressed note that begins a phrase on any beat other than the downbeat.
In the case illustrated here, the anacrucis is a quarter note (crotchet) and the rhythm is balanced by the final bar which has one quarter note less. The anacrusis could be any note (eight note, half note, etc.) or even more than one note (for example, 3 eighth notes). The strong accents would still fall on the first beat of the bar.