A canker is a sore or ulcer that develops in the mouth, usually on the inside of the cheek or lip. These small, painful spots make eating and talking uncomfortable for a few days until they heal. You might get a canker sore after accidentally biting your cheek or when you're stressed or run down. They're also called canker sores or aphthous ulcers.

The word can also describe a diseased area on a plant or tree where the bark becomes damaged and stops growing properly. An apple tree might develop a canker that weakens its branches. Gardeners watch for cankers because they can spread and seriously harm the plant.

In older writing, you might see canker used to describe anything that corrupts or destroys slowly from within, the way rust eats away at metal. A teacher might say that cheating is a canker in a classroom community because it gradually damages trust between students. This meaning suggests something that starts small but spreads its harmful effects over time, weakening whatever it touches.