component

A component is like the pure ingredients you would list on a recipe. In an alloy, the components are the pure chemical elements (or stable compounds) you mix together. Copper-nickel has two components, Cu and Ni; salt water has two, H2O and NaCl. Count the ingredients and you have counted the components.

More precisely, a component is one of the pure chemical species that make up the composition of the system, and the number of components C is the minimum number of independent species needed to describe every phase. Composition is normally given as the weight percent of each. A 70 Cu - 30 Zn brass has two components; a plain steel is usually treated as the two components Fe and Fe3C (or Fe and carbon).

The count of components feeds straight into the Gibbs phase rule and decides which kind of phase diagram you draw: one component gives a unary pressure-temperature diagram, two give a binary diagram. Do not confuse components (the ingredients) with phases (the physically distinct regions that actually form). Sugar water has two components but can be one phase (a clear solution) or two phases (solution plus undissolved sugar at the bottom).