Beat
The steady pulse in music, found by watching for sudden jumps in low-frequency energy.
See it in Dither playground. Turn on the microphone, play some music and set the band to Bass.
In plain words
Music has a pulse you can tap your foot to. That pulse is the beat. Kick drums and bass notes carry most of it. A computer can find beats by watching the bass level and noticing when it jumps suddenly above its recent average. Visuals can then flash, grow or change on every beat.