What's the best definition you've seen? I would like to use an existing government policy or national lab/academies report. Or, how could I improve these two draft definitions below?

To start, the US Energy Policy Act of 1992 still seems to be the authority on "alternative fuel." (Thank you EERE for making this list easy to find.)

Methanol, ethanol, and other alcohols

Blends of 85% or more of alcohol with gasoline

Natural gas and liquid fuels domestically produced from natural gas

Liquefied petroleum gas (propane)

Coal-derived liquid fuels

Hydrogen

Electricity

Fuels (other than alcohol) derived from biological materials (including pure biodiesel (B100))

P-Series

My first stabs at "drop-in" and "non-drop-in." (Thank you CAAFI for getting me started.)

Drop-in Alternative Fuel:

An alternative fuel that is completely interchangeable and compatible with a particular conventional (typically petroleum-derived) fuel. A perfect drop-in fuel does not require adaptation of the fuel distribution network or the vehicle or equipment engine fuel systems, and can be used “as is” in vehicles and engines that currently operate on that particular fuel. Some alternative fuels may become “drop-in” only after blending with conventional fuel to a certain prescribed proportion.

Non-drop-in Alternative Fuel:

An alternative fuel that is not completely interchangeable and compatible with a particular conventional (typically petroleum-derived) fuel. A non-drop-in fuel requires adaptation of (or special treatment within) one or more components of the existing fuel distribution network or the current fleet of vehicle and equipment engine fuel systems. Some alternative fuels must be carefully segregated from conventional fuels, while others may be safely blended with conventional fuels. Some alternative fuels may remain “non-drop-in” even after blending with conventional fuel.