Parallel reactions are those reactions in which the reactant can form one of two or more products. 

Parallel reactions are concurrent chemical reactions with at least one common reactant. The term is infrequently used when a common product results from different reactants.

The reactions in which a substance reacts or decomposes in more than one way are called parallel or side reactions.

Examples of parallel reactions are (1)nitration of phenol with the formation of ortho-, meta-, and Pra-nitrophenol (identical reactants) and (2) nitration of a benzene and toluene mixture (nitric acid is the common reactant).