What is Social Efficiency? Is it the Same as Allocative Efficiency?

Allocative Efficiency is also called Social Efficiency at times. This is because at an output level that is allocative efficient, the social surplus, which is the sum of both consumer surplus and producer surplus, of the economy is maximized. Because the scarce resources are used to reflect the preferences of the consumers, there is no deadweight loss in the economy. This in turn means that the social surplus is maximum.
There is no deadweight loss at this level of output because the marginal utility (and price) is equal to the marginal cost. Social efficiency could be different from allocative efficiency in case of externalities. Because in case of externalities, the private marginal benefits do not reflect the externalities and as a result the social marginal benefits are different from private marginal benefits.