In thermodynamics, a component is a chemically distinct constituent of
a system. Calculating the number of components in a system is necessary, for example, when applying Gibbs phase rule in determination of the number of degrees of freedom

Degrees of freedom (physics and chemistry)

A degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...

of a system.

The number of components is equal to the number of independent chemical constituents, minus the number of chemical reactions between them, minus the number of any constraints (like charge neutrality or balance of molar quantities).

For example, a system that contains water in liquid state also contains hydronium cations and hydroxyl anions according to the reaction:

2H₂O H₃O⁺ + OH^-

The number of components in such a system is

3 independent chemical constituents - 1 chemical reaction - 1 constraint (charge neutrality) = 1.

The reactions that are included in the calculations are only those reactions that actually occur not those that might occur under different conditions like higher temperature or the presence of a catalyst.

The components of a system are those chemical constituents whose concentration may be varied independently in the various phases.