Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant:  Human: When is a door not a door?

Assistant: When it's a window.

Human: What properties can door have that make a door not a door?

Assistant: 
A door is considered not a door when it does not fulfill the basic function and purpose of a door, which is to provide an opening or a barrier between two spaces. Some properties that could make a door not a door include:

Lack of a hinge mechanism to swing open or close
Inability to lock or secure the opening
Lack of appropriate size or shape to effectively block the opening
Absence of a handle or knob to open and close the door
Material that is not suitable for a door, such as a sheer curtain or a piece of cloth
In essence, if a structure or object fails to provide the basic functions of a door, it would not be considered a door. Perhaps a door which has been repurposed. For instance, an old door which has been made into a table. The tricky thing here is that there are two definitions in play, one for a large piece of wood or metal which can serve as a door, and one meaning anything that serves as a barrier between two spaces. A door cannot have properties that make a door not a door because a door is a door.
If it is, in fact, a window the prime property would be whether it was intended for people to pass through as its primary purpose.