A reaction in which two or more reactants combine to form a single product is known as a combination reaction. Combination reaction is also known as a synthesis reaction.
Let us consider X and Y as reactants then the general formula for combination reaction is,
X + Y → XY
Examples of combination reaction
2Mg + O2 → 2MgO
Ba + F2→ BaF2
2Na + Cl2 → 2NaCl