con·tam·i·nate
[v. kuhn-tam-uh-neyt; n., adj. kuhn-tam-uh-nit, -neyt]
noun
3.
something that contaminates or carries contamination; contaminant.
One could argue that maintainers of online dictionaries rush to canonize common grammatical errors, I suppose.
Just curious here but what is the first described meaning of the word where you found its definition. Was it described as a verb, or a noun?