If you smear a surface with an oily or sticky substance or smear the substance onto the surface, you spread a layer of the substance over the surface.

A smear is a dirty or oily mark.

To smear someone means to spread unpleasant and untrue rumours or accusations about them in order to damage their reputation.

A smear is an unpleasant and untrue rumour or accusation that is intended to damage someone's reputation.

A smear or a smear test is a medical test in which a few cells are taken from a woman's cervix and examined to see if any cancer cells are present.