Illustrated Glossary of Organic Chemistry

First order (1^st order): A rate expression involving the concentration of a single species to the first power: rate = k [species]. Also refers to a reaction or reaction mechanism having this rate expression.

This S_N1 reaction has first-order kinetics, because its rate expression involves only the concentration of tert-butyl chloride to the first power.

NaI Acetone

Rate = k [CH3Cl] [I-]

This S_N2 reaction has second-order kinetics, because its rate expression involves the concentration of methyl chloride to the first power times the concentration of iodide ion to the first power.