Downbeat

The first beat of a bar is called downbeat. It is usually stressed.

Upbeat

The upbeat is the never-emphasized pickup of a piece.

Backbeat

The expectation of the listener is that all emphasized notes are played „on the beat“. If the emphasis is deliberately shifted to the unstressed notes (accent shift), this is called backbeat.

Offbeat

Syncopation may also produce deviations from the expected beat, which is called offbeat. In this case, the meter is not changed per se, but the notes are executed offset to the beat. Even unstressed notes can be emphasized (accent shift). In Romanticism, this was called rubato.

Whereas offbeat phrasing in Jazz and much other Blues system-based music occurs as a regular part of the musical scheme, a composer of European music typically incorporates offbeat phrasing in an effort to introduce something out of the ordinary, a change from the regular beat. One might think of the latter as a musical special effect. However, blues-system music often uses off-beat phrasing as the foundation for its rhythmic structure … it is the fundamental beat upon which the music is built.

Richard J. Ripani: The New Blue Music: Changes in Rhythm & Blues, 1950-1999. Univ. Press of Mississippi 2009, S. 47