Rearview describes the mirror that shows a driver what is behind. Fixed to the windscreen, and adjusted before every journey.
The word goes before mirror almost always and is rarely used alone. The little tab underneath flips it to a dimmed setting for night driving, which a surprising number of drivers never discover. Modern cars are adding cameras that feed a screen in the same place. There is a fine figurative use about the past, since looking in the rearview mirror means dwelling on what is behind you.
1adjectiverearviewDescribing the mirror showing the view behind a vehicle.
“He caught sight of the police car in the rearview mirror.”
From rear + view.
Etymology from Wiktionary, CC BY-SA