What non-K-Pop fans think:
The Korean word used by younger males to address older females.
In K-Pop context:
The right for fangirls to gush over how adorable their idols are when the artistes address them as 'noona'.
The 'Noona' market, which characteristically is one that has a bigger spending power than regular fangirls, was what boy group SHINee was marketed for when they made their debut. The group became an instant hit with the older fan-girls with the song 'Noona, You're so Pretty'.