heartbeat

Short definition from Wiktionary

Noun

1. 1
   One pulsation of the heart; especially an irregular one, hence the emotion which causes it.
2. 2
   The rhythm at which a heart pulsates, a cardiac indicator.
3. 3
   A driving impulse or vital force; the vital center of something.
4. 4
   A very short space of time; an instant.
5. 5
   A periodic signal generated by hardware or software to indicate normal operation or to synchronize other parts of a system.