SN2 mechanism (S-substitution, N-nucleophilic, 2-second order) is one of the two limiting mechanisms of nucleophilic aliphatic substitution at saturated carbon. It is a one-step mechanism:
A nucleophilic aliphatic substitution at saturated carbon occurring via SN2 mechanism is caled an SN2 reaction. The rate law of an SN2 reaction is
rate = k [substrate][nucleophile]
According to the rate law, an SN2 reaction is second order overall, and the concentration of both substrate and nucleophile affect the rate of reaction, which is consistent with the single-step mechanism.
see also SN1 mechanism