Tarnish a. : To soil, or change the appearance of, especially by an alternation induced by the air, or by dust, or the like; to diminish, dull, or destroy the luster of; to sully; as, to tarnish a metal; to tarnish gilding; to tarnish the purity of color.

Tarnish v. i. : To lose luster; to become dull; as, gilding will tarnish in a foul air.

Tarnish n. : The quality or state of being tarnished; stain; soil; blemish.

Tarnish n. : A thin film on the surface of a metal, usually due to a slight alteration of the original color; as, the steel tarnish in columbite.

Example:
How do you remove Tarnish from silver clarinet keys without harming the wood?

Taxonomy
free encyclopedia, product, thin coat, search, reactive metals, metal, Unsourced, layer, occurrence, chemicals, Prevention, using steel, rub, dull surface, physical chemistry, Personal tools, Interaction, Toolbox.

Defination from the web:
Tarnish is a product of a chemical reaction between a metal and a nonmetal or compound, especially oxygen and sulfur dioxide. ... Using a thin coat of polish can prevent tarnish from forming over semi-reactive metals. ...