Coinflip is a term specifying a situation in poker when two players are all-in and do have approximately similar chances to win a hand. On a long run it’s possible to define a winner in such situations by flipping a coin: everybody has a 50% chance to win.

The name of the term comes from an English 'coin flip', which means tossing a coin, since chances to hit heads or tails equal 50%. In reality, equal chances to win in poker are quite rare, that’s why the situations when player’s hand equity are 40%/60% are also considered a coin flip (i.e., the weakest hand at the moment has at least 40% to win).

For example, you can call situation a coin flip, when players pushed all-in with pocket     and    . Their chances to win are about the same, despite the fact that a player with Jacks has a little bit more chance (~56%).

From a poker mathematics point of view, coin flip is a little minus EV solution on the long run (because of the rake). At a short run coin flips are significantly swinging the variance (by a simple reason that we are risking our whole stack, not having any clear advantage in equity over our opponent’s hand).

On a poker slang coin flip can be called just ‘flip’ or 'to make a flip'.