: something that floats in or on the surface of a fluid: such as
a : a device (such as a cork) buoying up the baited end of a fishing line
b : a floating platform anchored near a shoreline for use by swimmers or boats
c : a hollow ball that floats at the end of a lever in a cistern, tank, or boiler and regulates the liquid level
d : a sac containing air or gas and buoying up the body of a plant or animal
e : a watertight structure giving an airplane buoyancy on water