Telescope mirrors used to be made of metal, laboriously worked to the right shape. Metals tarnish, however, so that the labor -- or most of it -- had to be repeated periodically: Polishing off the tarnish changed the shape enough to matter. It was a big deal, in the nineteenth century, when new chemical processes made it possible to deposit a film of silver on glass: A tarnished mirror could be repaired simply by removing the remains of the old coating chemically, and depositing a new one. In the 1930s, the new process of aluminizing allowed deposition of a coating of aluminum on a mirror in a vacuum chamber, essentially by boiling aluminum vapor off a heated piece of that metal.
Aluminum is much more tarnish-resistant than silver, hence its advantage. Other metals and materials may similarly be deposited.

This definition is in context to Telescope. See more contextual defintions for Aluminizing.