A flexible device with a concave surface or similar empty space and a full, round outer lip that can use a partial vacuum to attach to some flat surfaces without easily falling or being pulled off.

Placing a suction cup against a suitable flat surface and applying pressure flattens the cup thereby forcing air or liquid from its void, and then releasing that pressure allows the suction cup to return to its original shape while creating a sealed pocket of low pressure. The pressure difference between the inside of the cup and the external atmosphere allows the suction cup to resist gravity and other forces.

They are usually a component of larger objects meant to hold them in place for convenient use, especially on dildos seen on e621, but some objects with suction cups are primarily intended to manipulate suction, such as plungers. The muscly suckers found on octupus and squid tentacles are also considered suction cups.