A substance or agent that removes color; a bleaching or whitening agent (American English spelling).
From decolorize + -r (agent noun suffix). The -er or -or suffix creates nouns for entities (usually things) that perform the action of the verb.
Notice how 'decolorizer' (American) and 'decoloriser' (British) sound identical despite different spelling—this shows that spelling conventions don't change pronunciation, which is why Americans and Brits can understand each other perfectly despite these 'z' vs 's' differences!
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