Barrier

In generative syntax, a barrier is a category which serves as a blockade for government and/or movement.

Comments
The original definition (from Chomsky 1986) is:

''g is a barrier for b iff (a) or (b): a. g immediately dominates d, d a Blocking Category for b    b. g is a Blocking Category for b, g =/= IP''

Link
Utrecht Lexicon of Linguistics