What is "side reaction"

A side reaction is a chemical reaction that occurs at the same time as the actual main reaction, but to a lesser extent. It leads to the formation of by-product, so that the yield of main product is reduced:

$\mathrm{A + B \ \xrightarrow{k_1} \ P_1}$

$\mathrm{A + C \ \xrightarrow{k_2} \ P_2}$

P is the main product if k> k. The by-product P is generally undesirable and must be separated from the actual main product (usually in a costly process).