Question

What do you mean by reactant?

Answer 1

it is written on the left side of reaction or we can say the substance which under goes reaction

Answer 2

Reactant are those substance which undergo a chemical reaction.
In a chemical equation they are written in Left Hand Side before the arrow(which donates reaction)
eg.-》 2Mg+O2 --> 2MgO (Reactants and in LHS i.e. Mg and Oxygen)