open door

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

• noun Unhindered admission or access.
• noun A policy whereby a nation trades with all other nations on equal terms.

from the GNU version of the Collaborative International Dictionary of English.

• Open or free admission to all; hospitable welcome; free opportunity.
• In modern diplomacy, opportunity for political and commercial intercourse open to all upon equal terms, esp. with reference to a nation whose policy is wholly or partially fixed by nations foreign to itself, or to territory newly acquired by a conquering nation.

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

• noun the policy of granting equal trade opportunities to all countries
• noun freedom of access