Mate is a word with several common meanings:

1. A friend or companion. In British and Australian English especially, people call their friends mates. “G'day, mate!” is a cheerful Australian greeting. When you say someone is your mate, you mean they're someone you enjoy spending time with and can count on.
2. An animal's partner. Scientists say that wolves mate for life, meaning they stay with the same partner. Birds often perform elaborate dances or songs to attract a mate.
3. A helper or assistant in certain jobs. A ship's first mate is the captain's main assistant who helps run the vessel. A plumber's mate helps the lead plumber carry tools and learn the trade.
4. In chess, to checkmate (often shortened to mate) means to trap the opponent's king so it cannot escape capture, winning the game. Players might say “That's checkmate!” or simply “Mate!”