A thermochemical reaction is a chemical reaction with an additional information of the heat of reaction. For example, this is the thermochemical reaction for the combustion of hydrogen gas:
H₂ (g) + 1/2 O₂ (g) → H₂O (l) ΔH = -285.8 kJ
The coefficient 1/2 represents the number of moles of O₂ needed to combust 1 mole of H₂ gas in order to yield one mole of water.