best friend
compound. A1 Beginner Oxford
compound. Your best friend is the one friend you like more than any other.
compound. A person's closest and most trusted friend.
She is my best friend.
He's going on holiday with his best friend from school.
Despite the years and distance, she remained his best friend and confidante, the one person who truly understood him.
Usage
Often used with a possessive pronoun like 'my', 'your', or 'his'.
Teaching tip
Contrast with 'close friend' (one of several) and 'acquaintance' (someone known casually) to teach degrees of friendship.
Pitfall
I have two bests friends.I have two best friends.Only the noun 'friend' becomes plural, not the adjective 'best'.