When I learned the term back in the early '90s due to anime, I learned it basically as "a scene or element added to a piece of media that isn't there to serve the narrative, but instead appeal to fans in a specific way". While some people assign that strictly to things related to sex, that can be "showy" alt outfits, panty shots, the ending of Legend of Korra, a cameo by a certain character, and plenty of other things.
I've seen no reason to change that definition over the course of my life, so that's how I'd still define it.