Syntactic Theory
Government
governs 
if and only if
i) is a governor; and
is a governor; and
ii) m-commands ; and
m-commands ; and
iii) no barrier intervenes between and .
and .
Maximal projections are barriers to government. Governors are heads.
There is a refined notion of government which is particularly important for Case-theory. It is the same as the one above, except that the notion of governing head is specified individually:
Lexical heads (V0, N0, P0, A0) and tensed I0 are governors.