any final competition to determine a championship
"The team will face off in an intense playoff match to decide who wins the championship."
A final game in a series needed to break a tie.
"The team had to win the playoff game to avoid being eliminated after the regular season ended in a tie."
In plain English: A playoff is an extra contest between top teams to decide who wins when regular games end with a tie.
"The team made it to the playoffs after winning every game in their division."
Usage: The term refers specifically to postseason games that determine a champion after regular-season ties, rather than any casual match or exhibition. It is often confused with "play-off" when used as an adjective modifying another noun, though the single-word form is standard for both roles in modern usage.
The word playoff comes directly from the phrase "play off," which originally meant to compete against someone else in a game or contest. It entered English as a single term to describe a specific tournament match that decides a winner after other games have been played.