Also known as: チクニー, 乳首オナニー, 乳首責め, 乳首イキ, 玩弄乳头, 乳首つねり, 乳首責め騎乗位, 捏乳头
Pinching a person's Nipples and rolling the tips between your fingers as if tuning the dial of a radio.
If the nipples are being pulled, it can be tagged Nipple Pull as well.
Use Nipple Rub if the person is "caressing" the nipple instead of pinching it.