Definition of flip
An flip refers to the image of the coin. If a coin is tossed, 50% of the time it will fall on heads, 50% of the time it will fall on tails, 50% of the time it will fall on tails. An flip in poker is a 50-50 situation. Most of the time it will be a all in-preflop situation. They say 50-50 to turn the round corners, but most of the time when we talk about an flip, it will be a 55-45 situation that will be considered an flip. If you've ever played poker tournaments, you know that sometimes it's crucial to push all in your stack with a hand that's even the slightest bit clean. So you wait for your moment, you're on the button with a pair of 5's, you push all in and your opponent calls with AT.
An flip = often 1 pair vs 2 overcards
Our example above is classic. This type of flip in tournament is common. To explain what an flip is in other words, we could say that it is a all inpreflop where one player has a pair and the other has 2 cards up (overcards) to the pair. In our example, the ace and the 10 that are above the pair of 5s.