Reactant
A reactant is a starting substance that is consumed in a chemical reaction; reactants are written on the left side of a chemical equation.
Definition
Reactants are the substances you start with in a chemical reaction, written on the left side of the equation, and they get used up as the reaction proceeds. A reactant is consumed as products form, its coefficient sets the ratio in which it combines with other reactants, and if one runs out before the others it is the limiting reactant, which stops the reaction. In reaction kinetics, rate is defined from how fast reactant concentrations fall, expressed as the negative of that decrease divided by the reactant's stoichiometric coefficient, and the limiting reactant determines the theoretical yield while the others remain in excess. In a reversible reaction, calling a species a reactant is just a statement about which direction the equation is written, since at equilibrium both reactants and products coexist. Catalysts and solvents, present but never consumed, are not reactants.
Example
Burning a candle consumes wax and the oxygen in the air as reactants, both disappearing as the flame burns down. In $2Mg + O_2 \rightarrow 2MgO$, two magnesium atoms react with every oxygen molecule, so $3$ magnesium atoms paired with only $1$ oxygen molecule leaves one magnesium atom unreacted, making oxygen the limiting reactant. The same idea scales up: reacting $10.0\text{ g}$ of $H_2$ ($4.96\text{ mol}$) with $64.0\text{ g}$ of $O_2$ ($2.00\text{ mol}$) in $2H_2 + O_2 \rightarrow 2H_2O$ requires $4.00\text{ mol}$ of hydrogen for the oxygen present, so oxygen is limiting, giving a theoretical yield of $4.00\text{ mol}$ ($72.1\text{ g}$) of water with $0.96\text{ mol}$ of hydrogen left over.
Key Insight
Reactants always sit on the left of the arrow, since left means "before" in a chemical equation. Industrial processes use that asymmetry on purpose, supplying one reactant in large excess to push equilibrium toward products and to fully consume the pricier reactant, so deciding which reactant ends up limiting is a design choice, not an accident.