The lack of opportunities available to an
individual,
household, or group due to their short-term situation. Social exclusion is a process that was first identified in the 1990's as the cleavage between the "haves" and "have-nots" became even more pronounced.
Social exclusion is what beget what Americans call the "underclass". In states with an institutional welfare system (contrast with the United States' residual system) social exclusion is particularly embarassing given the high level of universal benefit available to citizens.
Unlike workers during industrialization, the socially excluded do not have a positive rallying point around which to build political clout (collecting from the dole does not breed unity). Those best equipped to organize them, are those who leave the category. Thus social exclusion has become a political and economic topic with almost no input from those affected.
Unhappy American high school kids sometimes confuse social exclusion with ostracisation and a disjointed social scene. This colloquial view is not at all related to the academic definition of the term.