iron curtain

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The military, political, and ideological barrier established between the Soviet bloc and western Europe from 1945 to 1990.
• noun A barrier that prevents free exchange of ideas and information.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A barrier made of iron in the theatre, lowered between the stage and the auditorium for safety or to prevent communication.
• noun idiomatic Any impenetrable barrier.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an impenetrable barrier to communication or information especially as imposed by rigid censorship and secrecy; used by Winston Churchill in 1946 to describe the demarcation between democratic and communist countries