A government issued instrument that provides enterprises included in the ETS with the ability to emit one tonne of the regulated CO2e in accord with the conditions of the ETS. Typically, emission allowances are issued with ‘vintage’ designations which define the time period (typically, a particular year) in which the allowances can be first surrendered by enterprises to the ETS-administering entity. If allowed by the ETS, an allowance can be surrended in the defined year (vintage) or any year thereafter.