The flop is a term found in Hold 'em, Omaha, and a few other games that use community cards. It describes the first set of community cards that are dealt by the dealer into the center of the table. Typically, this is a set of three cards dealt at once, but it is possible for some games to define different-sized flops.
In most cases, and if given no other context, the flop describes the initial three community cards in either Hold 'em or Omaha.
Flop is also used as a verb, to describe "making a poker hand using your hole cards and the flop." A common example is for a player to say they "flopped two pair" if, after the flop, they had two pair. It can also be used in the present tense, as in "I hope I flop a straight!", or "I've been flopping full houses for three hands running!"