A chemical such as MTBE, ETBE, or ethanol added to a motor fuel. Since oxygen is essential to combustion, these can be added to gasoline in order to enable the fuel to burn more completely. More complete combustion results in fewer hydrocarbons in exhaust gases, which, in turn, reduces smog and similar pollutants.
Oxygenates do, however, have downsides. MTBE has been known to escape from underground gas tanks and contaminate groundwater. Its effectiveness in reduction pollutuion has been hotly debated, and has been decided that the negatives outweigh the advantages, and it is being phased out and replaced with oxygenates such as ETBE and ethanol.
Another downside, though mostly insignifcant, is the oxygenates have a slightly lower energy content than their hydrocarbon counterparts, which results in decreased engine output and/or increased fuel consumption.