As we all know, time is continuous. In traditional music, time's continuity is divided into units called beats. Beats (sometimes referred to as pulses) are nothing more than divisions of time.
Beats are not always regular - beats can also change speed. The fascinating thing about beats and speed is that as you listen to them being measured on a metronome, the sound (click) you hear marks only the beginning of the beat. It is the length of time between the clicks that determines the beat's speed. So, when you use the term "beat", think of the entire "duration" of the beat and not just the click.