Weak beat
Unaccented part of a bar, as opposed to the strong beat.
What it means
The weak beat is the unaccented portion of a bar that follows or precedes the strong, accented beat. In common time, the second and fourth beats are usually considered weak, the first (and, with reservations, the third) strong.
Older, mostly German, treatises sometimes translate this as the 'bad' beat, without any negative judgement intended; the term simply denotes the metrically weaker position within the bar.
Musical use
The distinction between strong and weak beats underlies metre itself and explains why an upbeat (anacrusis) begins on a weak beat, and why certain dissonances, such as passing or neighbour notes, tend to appear on weak beats. The weak beat should be distinguished from the upbeat proper, which is an independent preparatory gesture before the first strong beat of a phrase.
Performance practice
When practising, avoid unconsciously accenting weak beats; the natural alternation of strong and weak carries the musical pulse and should remain audible even with even note values.