Definition of coin flip
A 50%-50% situation. The expression coin flip refers to a coin (corner). If someone tosses a coin in the air and asks you if the coin fell on a coin flip and hides the result, you will have a 50% chance of getting the correct answer. This is the metaphor often used to talk about a game with a 50% chance of success or to talk about an all in with 50% equity preflop.
Further clarification on the coin flip
If a player is talking about an coin flip, he will often want to talk about a hand that went all-in preflop with 50% equity. The classic example of an coin flip is when a player calls an all in with AK and his opponent has a pair. If we want to be picky, we will say that the equity of the hands is not exactly 50%-50%, but that is what is meant by coin flip. Both hands will have roughly 50% equity each. Sometimes you won't have a choice to take corners flip. Imagine you're in a tournament and you have a small mat, say 5 blinds. The players have all folded down to the small blind that pushes all in (once again). You look at your cards and find a pair of 5's. Your opponent will almost always have 2 cards higher than your 5 for a 50% equity. You will then say that you picked a flip to collect chips.