Decomposition reaction are opposite to combination reactions. Because in a decomposition reaction, a single substance decomposes to give two or more substances.
Decomposition reaction:
AB + Energy → A + B
2H2O(l) → 2H2(g) + O2(g)
Combination reaction:
A + B → AB + Energy
2H2(g) + O2(g) → 2H2O(l) + Energy