Heartbeat

Noun

1. One pulsation of the heart; especially an irregular one, hence the emotion which causes it.

   "The policeman waited for a heartbeat in vain."

2. The rhythm at which a heart pulsates, a cardiac indicator.

   "If your heartbeat doesn't normalize soon, consult a doctor!"

3. A driving impulse or vital force; the vital center of something.

   "Music is the heartbeat of the people."

4. A very short space of time; an instant.

   "The ambulance arrived in a heartbeat."

5. A periodic signal generated by hardware or software to indicate normal operation or to synchronize other parts of a system.