When an object is inserted inside one's [[nipples]], thus the nipple being penetrated.

Can include fingers, needles, penises, tentacles and even other nipples.

Many examples can be found in the [[Taimanin Asagi]] series, especially in the [[Taimanin Asagi 2]] eroge and hentai OVA episode 1.
ニプルファック 乳首姦