A soil horizon is a unique horizontal layer of soil which can be identified as O, A, E, B, C, or R having distinct physical and chemical properties.

Definition of Soil Horizon
Soil horizon is a parallel layer of soil to the surface of the ground formed by the pedogenesis processes. Every horizon has a different appearance compared to one another.
Wikipedia
Wikipedia

Types of Soil Horizons
O horizon – Organic soil rich in decomposed plants.
Encyclopedia Britannica
Encyclopedia Britannica
A horizon (Topsoil) – A combination of mineral soil and humus.
biologyinsights.com
biologyinsights.com
E horizon – A layer where clay and organic matter are removed and is light in color.
Encyclopedia Britannica
Encyclopedia Britannica
B horizon (Subsoil) – A layer deposition of clay, oxides and mineral layers.
biologyinsights.com
biologyinsights.com
C horizon – The horizon where parent material of soil is found.
Encyclopedia Britannica
Encyclopedia Britannica
R horizon – Solid and compact bedrock layer.
Encyclopedia Britannica
Encyclopedia Britannica
How Soil Horizons Form
Soil horizons form as a result of the activities of weathering, leaching, biological actions, and accumulation of materials.
iere.org
iere.org

Soil Profile
Soil profile is a succession of horizons forming the soil profile that reflects on the formation of the soil.
Educational Resource
Educational Resource

Soil Horizons form the basis of soil classification system used in both scientific and practical purposes.