The phrase "unburned fuel" refers to the fuel that remains after a combustion process without being completely consumed. It means that some fuel has not been turned into energy and remains unused in its original form. Full definition
One finally admits, however, that «looking at (emissions standards requiring) 0.04 grams / mile of unburned fuel in a ported engine is difficult.» (wardsauto.com)
This produces a bolus of unburned fuel in the exhaust stream. (digitaltrends.com)
Something to keep in mind regarding diesel dragsters is that they do not use any emissions equipment to more effectively burn unburned fuel and oil contained in diesel fuel which is essentially kerosene. (mechanics.stackexchange.com)