heartbeat (Noun) — The rhythmic contraction and expansion of the arteries with each beat of the heart.
heartbeat (Noun) — A very short time (as the time it takes the eye to blink or the heart to beat). ex. "if I had the chance I'd do it in a heartbeat"
heartbeat (Noun) — An animating or vital unifying force. ex. "New York is the commercial heartbeat of America"