Two female who have became best of friends. Best female friend.
This term was used back in the 80's to discribe friendship between two women. But now in the 21st century its replaced with Best Friend Forever (BFF) or Best Friend (BF).
I can't wait to bosom to see my bosom buddie, we were so close as youngins.
Me and her were bosom buddies in high school.
My bosom buddy is getting married.