A heavy rhythmic emphasis on beats 2 and 4 within a 4-beat bar. Many rock drum patterns are based around the kick (bass) drum playing on beats 1 & 3, with the snare drum sounding the backbeat on beats 2 and 4. This rhythmic pattern was first heard in New Orleans jazz bands in the early 1900s; it then spread to blues, rhythm and blues and rock music.
Historically, in Western music (which originated in Europe and spread to America), beats 1 and 3 have been considered the “strong” beats. These are heard emphasized in music from Bach to Beethoven, and especially in military marches. Some music historians contend that the heavy 2-and-4 backbeat was originally a parody of military music, devised by African-American musicians who heard both Union and Confederate bands play marching music during the Civil War.