Strong beat
The accented part of a bar, as opposed to the weak beat.
What it means
Older music theory used the term strong beats for the accented beats of a bar, as opposed to the unaccented, weak beats in between.
In most meters, the first beat carries the strongest accent; in compound meters there are also secondary strong beats, such as the third beat in common time, which is accented but more weakly than the first.
Musical use
The distinction between strong and weak beats forms the basis of a piece's metrical order: it creates the sense of pulse and regularity that performers and listeners orient themselves by, and it determines how naturally accented notes feel within a phrase.
Composers also deliberately work against the expectation of fixed strong beats: syncopations shift accents onto what would otherwise be weak beats, creating tension and rhythmic drive, because the ear perceives the displacement against the expected meter.
The strong beat should be distinguished from the upbeat, an unaccented anacrusis placed before the first strong beat of a piece or phrase, and from syncopation, which shifts an accent precisely away from a strong beat onto a weak one.
Performance practice
When practicing, bring out strong beats gently, for instance through slightly more weight or length, without stamping them out mechanically. Especially with syncopations, make the displacement against the expected strong beat clearly audible rather than smoothing it over in performance.