From Academic Kids
In music, a pulse is an unbroken series of distinct yet identical periodically occurring short stimuli perceived as points in time (DeLone et. al. (Eds.), 1975, chap. 3). Ideally, this is opposed to a series of identical but aperiodically occurring stimuli, a series of periodically occurring yet otherwise differentiated stimuli, or an uninterrupted stream of sound (such as a drone). A pulse may be an unheard event underlining a piece or metric level, thus the tempo of the piece is how fast the pulse is running, implied by the performer or listener based on context or past experience, or audible such as in Terry Riley's In C.
Non-ideal pulses varied according to strength or accents, which produce two or three pulse pulse groups (anything larger being a combination), strong-weak and strong-weak-weak (ibid). In fact, given an ideal pulse, the most probable reaction for one to have is to perceptually group or differentiate the beats. A pulse which became to fast would become a drone, a pulse that is to slow becomes isolated sounds. A pulse that is regularly accented is a meter. An isochronal or equally spaced pulse on one level that uses varied pulse groups (rather than just one pulse group the whole piece) create a pulse on the (slower) multiple level that is non-isochronal (a stream of 2+3... at the eighth note level would create a pulse of a quarter note+dotted quarter note as its multiple level).
See: beat (music).