Is magnesium a reactant or product?

It depends on the chemical reaction taking place.

magnesium + oxygen -> magnesium oxide

In equation 1 it would be a reactant.

magnesium oxide -> magnesium + oxygen

In equation 2 it would be a product.

If magnesium is on the left-hand side of the equation it is a reactant.

If magnesium is on the right-hand side of the equation it is a product.

From Wikipedia:

"The substances initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants."