Exothermic reaction.
\[\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + heat}\]
When quicklime (calcium oxide) is added to water, slaked lime (calcium hydroxide) is formed and a large amount of heat is given out; the water may even start boiling. Since heat is released, it is an exothermic reaction. It is also a combination reaction.