Reductive aroma
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"Reductive" aroma refers to the presence of hydrogen sulfide, which is formed by yeast as a chemical reduction of sulfite as part of its normal metabolism.
Chemical "reduction" is the opposite of chemical "oxidation". This is referred to as reduction-oxidation chemistry, or "redox", and it most often deals with the transfer of oxygen atoms, or sometimes just electrons to change a compound's "oxidation state". For every compound in a reaction that is reduced, there is another that is oxidized, and vice versa.
Refer to Hydrogen sulfide for a guide to preventing and removing a reductive aroma in wine or beer. For more information about redox chemistry in general, refer to a chemistry textbook, or "Redox" on Wikipedia.