In organic reactions what is mend by addition reaction?

An addition reaction is an organic reaction where two or more molecules combine to form a larger one.These reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon-carbon double bonds(alkenes) or with triple bonds(alkynes). Molecules containing carbon—hetero double bonds like carbonyl (C=O) groups, or imine (C=N) groups, can undergo addition as they too have double bond character.An addition reaction is the opposite of an elimination reaction.

Addition of chlorine to ethylene