Bestie is the most common informal term for "best friend" in English. It is a shortened form of "best friend," and is often used in a playful or affectionate way. The term is gender-neutral and can be used to refer to both male and female friends.
Here are some other informal synonyms for "best friend" in English:
BFF (best friends forever)
Sidekick
Wingman/wingwoman
Partner in crime
Bud
Pal
Mate
The more formal term for "best friend" is simply best friend. This term is typically used in more serious or formal contexts.
Here are some examples of how to use "bestie" in a sentence:
"Sarah is my bestie. We've been friends since we were kids."
"I'm going to the movies with my bestie tonight."
"I can't imagine my life without my bestie."
I hope this helps! Let me know if you have any other questions.