Noona (누나) is a Korean honorific. As stated on the Dramabeans website, “Traditionally, noona is what a boy calls his older sister, from childhood all the way through adulthood.

It’s also a term indicating friendship, or an informality between kids whose families are really close. Between kids, noona is simply something for a boy to call a girl who is older than him. But noona can also be used between adult friends, especially if the age gap is bigger and the relationship is close, as opposed to the more formal “Proper-name-sshi.”

The use of this term is also widespread among the K-pop fandom. It can also be romanized as “nuna”.