Thayer's Greek Lexicon

 1. a door
  1) the vestibule
  2) used of any opening like a door, an entrance, way or passage into
  3) in a parable or metaphor
1c1. the door through which sheep go in and out, the name of him
who brings salvation to those who follow his guidance
1c2. "an open door" is used of the opportunity of doing
something
1c3. the door of the kingdom of heaven (likened to a palace)
denotes the conditions which must be complied with in
order to be received into the kingdom of God