Definition of flip
A flip refers to the image of a coin. If you toss a coin, 50% of the time it will come up heads, 50% of the time it will come up tails. A flip in poker is a 50-50 situation. The majority of the time, it will be an all in preflop situation. We say 50-50 to turn corners, but most of the time when we talk about a flip, it will be a 55-45 situation that will be considered a flip. If you've ever played tournament poker, you know that sometimes it's crucial to push all in with a decent hand. So you wait for your moment, you're on the button with a pair of 5s, you push all in and your opponent calls with AT.
One flip = often 1 pair vs 2 overcards
Our example above is classic. This type of flip in tournaments is common. To explain what a flip is in other words, we could say that it is an all in preflop when one player has a pair and the other has 2 cards overcards to the pair. In our example, the ace and the 10 over the pair of 5s.