door (n.)

"movable barrier, commonly on hinges, for closing a passage into a building, room, or other enclosure," c. 1200, a Middle English merger of two Old English words, both with the general sense of "door, gate": dor (neuter; plural doru) "large door, gate," and duru (fem., plural dura) "door, gate, wicket." The difference (no longer felt in Old English) was that the former came from a singular form, the latter from a plural.

Both are from Proto-Germanic *dur-, plural *dures (source also of Old Saxon duru, Old Norse dyrr, Danish dr, Old Frisian dure, dore, dure, Old High German turi, German Tür). This is from PIE root *dhwer- "door, doorway."

Middle English had both dure and dor; the form dore predominated by 16c. but was supplanted later by door. The oldest forms of the word in IE languages frequently are dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves.

Figurative sense of "means of opportunity or facility for" was in Old English. Phrase door to door "house to house" is from c. 1300; as an adjective, in reference to sales, by 1902.

A door is what a dog is perpetually on the wrong side of. [Ogden Nash]

Definitions of door

door (n.)
a swinging or sliding barrier that will close the entrance to a room or building or vehicle;
he slammed the door as he left
he knocked on the door
door (n.)
the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close;
Synonyms: doorway / room access / threshold
door (n.)
anything providing a means of access (or escape);
we closed the door to Haitian immigrants
education is the door to success
door (n.)
a structure where people live or work (usually ordered along a street or road);
they live two doors up the street from us
the office next door
door (n.)
a room that is entered via a door;
his office is the third door down the hall on the left