Definition of side mirror in US English:

side mirror

noun

  • 1A mirror fixed or positioned to the side of something, especially a window or larger mirror.

  • 2Specifically (chiefly North American) a mirror projecting from the side of a motor vehicle, which allows the driver to see what is behind and to one side.

Origin

Mid 18th century; earliest use found in John Wilkes (1725–1797), politician.