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

An element or compound that enters into a chemical reaction is called a reagent, while the elements or compounds that result from a chemical reaction are called products. Reagents can be further defined as reactants, catalysts or solvents.

A reactant is a reagent that is consumed during the reaction. For example, in the reaction NaCl + AgNO3 --> NaNO3 + AgCl, the reactants are NaCl and AgNO3. A solvent is used to help another chemical dissolve, but it is not itself consumed by the reaction. A catalyst is also not consumed by a reaction, but it instead is used to force a reaction between other elements or compounds to take place.