This is retouching. It denotes the act of fixing undesirable aspects of a work. In the case of adult-oriented art, it can also mean adding black censor bars and mosaics.

Heeey, Kazaguuuumo! Spread your lips a bit, will yoooou! I need a reference, okaaaay!

No, no, you can still see bits and pieces through the censor! If you aren't okay with the front, spreading your back hole works toooo!

The 'back' isn't considered a sexual organ if there's no penetration, so in those cases not doing any retouching is fine too.

Aww, it didn't work...