A substance that removes oxygen from something, the opposite of an oxidant.
From dis- + occident (not standard) or dis- + oxidant. This is a technical term combining the prefix dis- with occident or a variant of oxidant to describe deoxygenating agents.
This is an extremely obscure term that appears in some scientific dictionaries but is almost never used in practice—chemists simply say 'reducing agent' instead. It's a wonderful example of how scientific terminology evolves, with some proposed words dying out because they don't fill a real need.
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