A piece of cloth covering a window, bed, etc. to offer privacy and keep out light.
"She drew the heavy velvet curtain across the window to block out the morning sun."
In plain English: A curtain is a piece of cloth that hangs from above to cover a window for privacy, light control, or decoration.
"She pulled back the heavy velvet curtain to let in some morning light."
Usage: Use the noun form when referring to the fabric itself that covers windows or beds for privacy and darkness. As a verb, it specifically describes the action of hanging this covering over an opening.
To cover (a window) with a curtain; to hang curtains.
"She carefully pulled the heavy velvet fabric across the window to darken the room."
In plain English: To curtain something means to cover it with curtains so you can't see through it.
"She curtained off the messy kitchen with a sheet while she cleaned up."
The word comes from the Old French cortine, which was borrowed from Late Latin cōrtīna. The original meaning was simply "curtain."