Heartbeat
WordNet
noun
(1) An animating or vital unifying force
"New York is the commercial heartbeat of America"
(2) The rhythmic contraction and expansion of the arteries with each beat of the heart
"He could feel the beat of her heart"
(3) A very short time (as the time it takes the eye blink or the heart to beat)
"If I had the chance I'd do it in a flash"