The interior of a building; the space inside buildings generally.
"The sudden storm forced everyone to retreat indoors, seeking shelter from the heavy rain."
In plain English: Indoors is any space that is inside a building and protected from the weather outside.
"The children played indoors for safety during the storm."
Inside, into, or within a building.
"Please bring your umbrella inside so we can stay indoors while it rains."
In plain English: Indoors means inside a building rather than outside.
"Please come inside and sit indoors while it rains outside."
Usage: Use "indoors" as an adverb to describe location without needing a following preposition like "in." Avoid confusing it with the noun form when you intend to modify a verb directly.
The word indoors began as a phrase combining the preposition in with the noun doors, which was itself an evolution of the older expression "within doors." This term traveled into English to describe locations situated inside buildings rather than outside them.