Scenes of characters changing clothes shown in part or entirely for the purpose of fan service.
Scenes featuring characters getting changed, dressed, or undressed, often with close-up or panning shots to titillate. Seen often in anime and manga but not limited to it, the Gratuitous Changing Scene is seen in all kinds of visual entertainment media aimed at older teens and adults.
Anime and manga featuring fan service strive to entice and titillate fans with the sex appeal of the characters. One of the easiest ways to do this is the Gratuitous Changing Scene. Generally set in locker rooms/changing rooms, bedrooms or bathrooms, characters are shown changing clothes after waking up, before gym class at school, before going swimming, before bathing and so on. The Gratuitous Changing Scene is often longer than arguably necessary and usually features close-ups, slow panning shots and shots from various different angels.
Specifically, Gratuitous Changing Scenes almost always feature panty shots. They're also the perfect opportunity for gainaxing, skinship groping, breast envy, wardrobe malfunctions and other related shenanigans.
In censored anime broadcasts, convenient censorship is used heavily in these scenes. Additionally, anime anatomy is very common as well, either instead of convenient censorship or in tandem. A typical mixture of the two forms of censorship sees objects, hair, shadows, light or steam concealing nipples while genitals are simply not drawn.
It should be noted that not all changing scenes are there for the purpose of fan service. In some instances, changing scenes are completely innocent or just generally done more tastefully. In other cases, changing scenes have some kind of plot significance. For example, a character may be shown in a state of undress, disgusted with their own body. Instances like these usually aren't considered fan service and thus don't fall under this specific concept.