The Flop is defined as the first three cards turned over in any poker game. These three cards the community cards are shared cards by each player.
I remember about 15 years ago when I first started playing live poker at cardrooms in California; I heard the term FLOP. I was playing in my first Texas Holdem Live poker game; and after the dealer turned over the first three cards. I realized I had a King High Straight Flush. I had the Queen Ten of Hearts and the dealer turned over the King, Jack, and Nine of Hearts. They knew I was just a novice player and I will never forget how there was a bet and a raise before the action got to me. I had to look at the cards twice because I realized there was no way on earth I cloud loose this hand.
Unfortunately I tried to raise again after putting enough chips in the pot and I was accused of something called a String Bet. A string bet is a bet when the player seems like he is just calling but then decides to raise and is common for new players.
But nonetheless, after I tried to raise the pot again and should have just remained calm and only called, the poker shark at the table asked me "Did you hit the flop"?
And when they explained to me what the FLOP was I said "Slightly". Well fortunately another heart hit the board and one of the aggressive players was holding the Ace of Hearts, which made the hand pretty interesting. It was the first really big hand I ever won and from then on out, I understood the meaning of "Catching the Flop". And it was all to soon that I had a better understanding of the term "Missing the Flop".
But what many poker players must understand is that after the Flop, the hand is half way done. When the post flop better round is complete the hand is at the halfway point. Most players will fold after the flop compared to after the next two community cards are revealed.
This is why you will always hear the Term "Preflop" and "Postflop".