heart·beat 
(härtbēt′)
(härtbēt′)n.
1.
a. A single complete pulsation of the heart.
b. The general nature of the heart's contractions or the rate at which they occur: the baby's heartbeat; a fast heartbeat.
2. A vital force or driving impulse: Broadway is the heartbeat of the New York City theater world.
3. An instant: I'll be there in a heartbeat.