In geology, a horizon refers to either a bedding surface where there is marked change in the lithology within a sequence of sedimentary or volcanic rocks, or a distinctive layer or thin bed with a characteristic lithology or fossil content within a sequence. In the interpretation of seismic reflection data, horizons are the reflectors (or seismic events) picked on individual profiles. These reflectors represent a change in rock properties across a boundary between two layers of rock, particularly seismic velocity and density.
Marker horizons are stratigraphic units of distinctive lithology (different from the bulk of the sequence) with a wide geographical extent that are used in stratigraphic correlation. Layers of tuff (lithified volcanic ash) are often used for this purpose.
An event horizon is a bed that marks a geological event, such as an earthquake or a meteorite impact. It is the basic unit used in event stratigraphy.
The fundamental unit of Russian stratigraphy, the gorizont, can be anglicized as "horizon". However, this concept is not equivalent to the term used in western geological systems. While the western term "horizon" pertains to a small lithological section within a geological formation, a gorizont is a broad biostratigraphic unit. It may encompass several "svitas" (lithological units equivalent to a formation). Both gorizonts and svitas are also considered chronostratigraphic units (correlated with a distinct time interval), while western geologists have separate chronological and stratigraphic systems.