An S2 reaction occurs when a nucleophile displaces a leaving group residing on a molecule from the backside of the leaving group.  This displacement or substitution results in the formation of a substitution product with inversion of stereochemical configuration.  The nucleophile forms a bond with its lone pair as the electron source.   The electron sink which ultimately accepts the electron density is the nucleofuge (leaving group), with bond forming and bond breaking occurring simultaneously at the transition state (marked with a double-dagger). The rates of S2 reactions are dependent on the concentration of the haloalkane and the nucleophile.
Because an S2 reaction proceeds with the substitution of a leaving group with a nucleophile, the S designation is used.  Because this mechanism proceeds with the interaction of two species at the transition state, it is referred to as a bimolecular process, resulting in the S2 designation. An S2 reaction is a concerted process, which means that the bonds are breaking and forming concurrently. Thus, the electron movement shown by the arrow pushing is happening simultaneously. An S2 reaction has one step.