\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.