Rust

noun

1. The deteriorated state of iron or steel as a result of moisture and oxidation.

   "The rust on my bicycle chain made cycling to work very dangerous."

2. A similar substance based on another metal (usually with qualification, such as "copper rust").

   "aerugo. Green or blue-green copper rust; verdigris. (American Heritage Dictionary, 1973)"

3. A reddish-brown color.
4. A disease of plants caused by a reddish-brown fungus.
5. Damage caused to stamps and album pages by a fungal infection.