Rate of Reaction Definition

For a reaction: $$ aA + bB \rightarrow cC + dD $$

$$ \mathrm{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt} $$

Defined as change in concentration per unit time, normalized by stoichiometric coefficient.