What is a neat liquid?
A pure substance (element or compound) that is in the liquid phase.
A liquid can be described as "neat" if it is a pure substance (a single compound, or a single element) that happens to be in the liquid phase.
For example, pure anhydrous ethanol is a "neat liquid". Pure benzene, with nothing else added to it, is also a "neat liquid", as is pure water.
The following, however, are not "neat": (a) vodka (a mixture of ethanol and water, essentially), (b) motor oil (it contains various different molecular types), (c) aqueous solutions (they contain water + other substances dissolved in it).