B and C from the above equations usually represent different compounds. However, they could also just be different positions in the same molecule.
A side reaction is also referred to as competing reaction[2][3] when different compounds (B, C) compete for another reactant (A). If the side reaction occurs about as often as the main reaction, it is spoken of parallel reactions[4] (especially in the kinetics, see below).
Also there may be more complicated relationships: Compound A could reversibly but quickly react to substance B (with speed k1) or irreversible but slow (k1> k-1 >> k2) to substance C: