An alloy is the purposeful mixture of two or more elements of which one being in largest proportion is called base metal, and other elements are called alloying elements. However, Steel, containing Iron and carbon, is not referred to as an alloy. But if elements are added other than Iron and carbon, the Steel is known as alloy Steel.
Thus all the Steels, in addition to Iron and carbon-containing other elements such as; nickel, chromium, manganese, silicon, vanadium, molybdenum, tungsten, sulfur, phosphorus, etc., are called alloy Steels.
The purpose of alloying is to improve the properties of Steel, like imparting the fine grain size, to improve the hardness, toughness, strength, corrosion resistance, etc. These are Steels made with the addition of a definite proportion of a selected element or elements in addition to carbon at the manufacturing stage.