Source : Webster's Revised Unabridged Dictionary (1913)

Open door \O"pen door\
      (a) Open or free admission to all; hospitable welcome;
          free opportunity.

                She of the open soul and open door, With room
                about her hearth for all mankind. --Lowell.
      (b) 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. In
          this sense, often used adjectively, as, open-door
          system, open-door policy, etc.

                The steps taken by Britain to maintain the open
                door have so far proved to be perfectly futile.
                                                  --A. R.
                                                  Colquhoun.

Source : WordNet®

open door
     n 1: the policy of granting equal trade opportunities to all
          countries [syn: {open-door policy}]
     2: freedom of access; "he maintained an open door for all
        employees"