What is the common name for iron oxide?

Rust is the common name for iron oxide. The most familiar form of rust is the reddish coating that forms flakes on iron and steel (Fe2O3), but rust also comes in other colors including yellow, brown, orange, and even green! The different colors reflect various chemical compositions of rust.

Rust specifically refers to oxides on iron or iron alloys, such as steel. Oxidation of other metals has other names. There is tarnish on silver and verdigris on copper, for example.