A bog is a mire that accumulates peat, a deposit of dead plant material—often mosses, and in a majority of cases, sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, quagmire, and muskeg; alkaline mires are called fens. They are frequently covered in ericaceous shrubs rooted in the sphagnum moss and peat. The gradual accumulation of decayed plant material in a bog functions as a carbon sink.

Bog is a wetland of mosses or lichens over waterlogged peat.