What's the word for the ass-shaped bump on a couch/sofa?

[Hanae]

I heard it in The Simpsons. Homer loves his couch because of the bump that perfectly fits his ass. Could anyone tell me the word?
I mean: What's the word for the arse-shaped bump on a couch/sofa?
I heard it in The Simpsons. Homer loves his couch because of the bump that perfectly fits his buttocks. Could anyone tell me the word?

 

[karma]

the indention.

 

[soolkimj]

Ass mold

 

[amanda]

An ass-groove.