What are mediating institutions?

In the most general sense "mediating institutions" are institutions that convey the norms and values of a society to individuals, so as to socialize them and integrate them into that society. In modern society, these might include churches, voluntary associations, schools or the mass media. They are referred to as "mediating" because they act as intermediaries between the ruling class that supposedly establishes societal norms and the masses who are expected to fall into line with those norms.