What is a chimney?

When most people hear the word chimney they picture the traditional masonry chimney. While that is a correct answer, the actual definition is;

A chimney is a structure that provides ventilation for hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney effect.

masonry chimney anatomy
Class A Double Wall Pipe is also a chimney.