1. Any of various powdery or scaly reddish-brown or reddish-yellow hydrated ferric oxides formed on iron and iron-containing materials by low-temperature oxidation in the presence of water.
2. Any of various metallic coatings, especially oxides, formed by corrosion.
3. A stain or coating resembling iron rust.
4. Deterioration, as of ability, resulting from inactivity or neglect.
5. Botany