1. n. [Geology]

An informal term used to denote a surface in or of rock, or a distinctive layer of rock that might be represented by a reflection in seismic data. The term is often used incorrectly to describe a zone from which hydrocarbons are produced.

See related terms: hardground

2. n. [Geophysics]

An interface that might be represented by a seismic reflection, such as the contact between two bodies of rock having different seismic velocity, density, porosity, fluid content or all of those.

See related terms: fluid contact,  lithologic contact,  reflection