A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation (horizonation) is a function of a range of geological, chemical, and biological processes and occurs over a long period of time. Soils vary in the degree to which horizons are expressed. Relatively new deposits of soil parent material, such as alluvium, sand dunes, or volcanic ash, may have no horizon formation, or only the distinct layers of deposition. As age increases, horizons generally are more easily observed. The exception occurs in some older soils, with few horizons expressed in deeply weathered soils, such as the oxisols in tropical areas with high annual precipitation.

The term 'horizon' describes each of the distinctive layers that occur in a soil. Each soil type has at least one, usually three or four horizons and these are described by soil scientists when seeking to classify soils (Soil-Net). Horizons are defined in most cases by obvious physical features, colour and texture being chief among them. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, ie, ‘coarser’ or ‘sandier’ than the horizons above and below.