What do you call the process when an element or compound that enters into a chemical reaction?

The elements or compound that enters into a chemical reaction are reactants or reagents.

Explanation:

A reagent or reactant is, in chemistry, any substance that interacts with another in a chemical reaction that gives rise to other substances of different properties, characteristics and conformation, called reaction products or simply products. Because they are chemical compounds, the reagents can be classified according to many variables: physical-chemical properties, reactivity in chemical reactions, characteristics of the use of the reagent.