Reactant
Definition
A starting substance in a chemical reaction: what gets used up as the reaction proceeds.
Technical definition
A reactant is a chemical species consumed during a chemical reaction, written on the left side of a chemical equation. Its amount, together with the reaction stoichiometry, determines how much product can form.
Example
In 2H2 + O2 -> 2H2O, hydrogen and oxygen are the reactants.
Common confusion
Reactant is not always the same as reagent. A reactant is a species in the balanced equation, while a reagent often means any substance added to cause a reaction, including catalysts and solvents that never appear in the equation.