A scrimmage is a practice — not an official game — in American football, hockey, and in other team sports.
You might think of football when you hear the word scrimmage, because the ball is placed on the "line of scrimmage" at the start of a play. A scrimmage is also a practice between two squads. You can also use scrimmage as a verb for other types of practicing. A basketball player might scrimmage by shooting one hundred free throws, for example. Originally, scrimmage was a variation on the word skirmish, and meant "a confused struggle between players."