Peat is a dark fibrous material created when 'decomposition fails to keep pace with the production of organic matter'. It is the first stage of transformation of plant matter into coal. Peat is developed under specific conditions; a water-saturated environment, absence of oxygen or nutrients, high acidity or low temperatures. Peat can nevertheless be found in many types of wetlands. Its structure ranges from more or less decomposed plant remains to a fine colloidal mass.

The following definitions are common*:

• Mire: Peatland where peat is currently being formed
• Peat: Sedentarily accumulated material consisting of at least 30% (dry mass) of dead organic material
• Peatland: An area with or without vegetation with a naturally accumulated peat layer at the surface
• Suo: A wetland with our without a peat layer dominated by a vegetation that may produce peat
• Wetland: An area that is inundated or saturated by water at a frequency and for a duration sufficient to support a prevalence of vegetation typically adapted for life in saturated soil conditions

* "Wise Use of Mires and Peatlands" by Donal Clarke and Hans Joosten (2002)