Rust is a product of corrosion of iron caused by a reaction with air and water. Carbon dioxide from the air dissolves in water to form an acid solution which attacks the iron to form iron (II) (ferrous) oxide. This is then oxidized (see oxidation by oxygen in the air to form reddish-brown iron (III) (ferric) oxide. Rusting may be prevented by coating the iron with zinc, a process called galvanizing, when the zinc is preferentially attacked by the acid solution.

In botany, rust is a group of fungi that live as parasites on many kinds of higher plants. The rust-red color of their spores gives them their name. Rusts are of particular concern for the damage they do to cereal crops, such as wheat and barley, and to several fruits and vegetables. They have complex life cycles that involve growth on more than one host plant.