Dr. Pat is right--it's a premature ventricular contraction
(PVC). They are also known as extrasystoles
and ectopic beats.
Every cell is the heart is capable of producing pacemaker activity, and when you have a PVC, a couple of rogue cells in the ventricles (the lower chambers of the heart) decide to produce an "extra heartbeat," thereby interrupting your heart's normal rhythm. As a result, there is a pause
as your heart's main pacemaker (the SA node) as it "resets." You might have to stay away from potential triggers, some of which are mentioned below.
Fear not, for they are harmless, but are a pain in the ***. I had to give up caffeine, alcohol, chocolate, and weed...however, I still sneak them into my diet occasionally, except the weed.
I hate them.