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1.
Immediately, at once, right away, instantly.

A very short time as denoted by the time it takes for the heart to beat.

Similar phrase: in the blink of an eye.

Oooh, i miss you in a heartbeat. Oooh, i miss you right away.
(Lyrics: Def Leppard - Miss You In A Hearbeat)
by yaeyae October 19, 2006