Show Me the Door

I've been a carpenter-mason all my grownup years, and something's been bugging this brain between my ears; I've asked experts in architecture and furniture, but the more they explain, all the more I'm unsure; I must have been to them either a pest or a bore, but then my question simply is: "What is a door?" Some say it's that panel or board that's hinged to the wall that you push or pull, swing or knock on when you call; With knob and key, with lock and bolt to secure the place, yet others say a door is but that vertical space through which you can walk or dash away from pole to pole; as a passage through a wall, a door is just a hole. But without a panel, you'll be knocking on nothing, and if you do, folks will think you're nuts or something; But then without that gap or hole we call empty space, that wall will flatten out that nose upon your face; Today, I don't ask that silly question anymore, for when I do, I'm shooed away and shown the door !