Oxidation or discoloration, especially of a decorative metal exposed to air.
1918, Hannah Teresa Rowley, Mrs. Helen Louise (Wales) Farrell, Principles of Chemistry Applied to the Household
Precipitated calcium carbonate, a very fine powdery form, is used as a basis for many tooth powders and pastes. As whiting it finds a wide use in cleaning metals of their tarnishes.
verb
(intransitive) To oxidize or discolor due to oxidation.
Careful storage of silver will prevent it from tarnishing.
(transitive) To compromise, damage, soil, or sully.
He is afraid that she will tarnish his reputation if he disagrees with her.
(intransitive, figurative) To lose its lustre or attraction; to become dull.
(copyright law) To use a sign, image, expression, etc. sufficiently close to a trademarked one that it brings disrepute to it.