What is the name of a chemical reaction whereby the initial volume of the reactants conmbined is reduced after the reaction.

Well, I think the easiest reactions that involve this are condensation reactions, where water is formed.

In the most famous one , you have two gases combining, hydrogen and oxygen, forming water.

4H_2 + O_2 \rightarrow 2H_2O

In the balanced reaction, you have 5 units of volume at the start, and only 2 at the end, provided the water is in the gaseous state.

Unknown008's right. In fact, almost any combustion reaction changes the volume of the chemical species involved. Most hydrocarbons are liquids or solids. Of course, some are gases (methane, for example). When they burn, water and carbon dioxide is formed. This normally results in some sort of change in physical properties and volume of material.

I don't think I know general name for a reaction where the volumes of the products differs from that of the reactants (I think this is what you meant to say). Maybe we could make one up: How about a volume-morphing reaction?