Hello!
The answer is: reactants... or can be called reagents too.
Why?
In every chemical reaction, we have the reactants. The reactants are the starting materials which after a chemical reaction will be transformed in products. After every chemical reaction, we have the products, but we also can have reactants (it happens when not all the reactants are used).
For example, given the following reaction:
NH3+ 2O2 → HNO3 + H2O
Reactants are: NH3 and 2O2
Products are: HNO3 and H2O
Have a nice day!