An unconformity is a stratigraphic feature that is formed by broad erosion of an area causing a significant gap to occur in the stratigraphic record. Generally, development of an unconformity is accompanied by or preceded by uplift of the rock units or strata that will be eroded, and/or subsidence or fall of sea level, thus exposing rock units or strata to erosion.

An unconformity can consist of two hypothetical parts: the hiatus and the erosional vacuity. The hiatus is the amount of stratigraphic record removed during erosion and the erosional vacuity is the amount of stratigraphic record that might have been deposited during the time erosion was occurring. The sum of these parts is referred to as the lacuna, or total missing stratigraphic record. An unconformity is commonly referred to as a significant gap in the chronostratigraphic record as well.

Unconformities can be classified according to their...