Chiefly British English.
That one person in your life who is like a sibling. You'll do anything for them, literally. They understand you, you understand them and you're always happy when you're around them. You just click.
Similar to best friend
, but you can only really have one best mate, otherwise they are probably just good friends.
My best mate is like a brother to me, and I hope we are best mates for life.