A buddy is a good friend. Your best buddy in elementary school may still be a good buddy when you graduate from high school.
If you love to hang out with a certain friend, she's your buddy. You could also call her your pal or your chum. Sometimes when kids are new in school, a teacher will ask another student to be their buddy — to help them feel comfortable and welcome. The word first appeared in the mid-19th century in the US, either as slang for brother or from the British butty, or "companion."