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vapor: The gas given off by a substance that is liquid (or solid) at normal temperatures and pressures.
vapor density: The ratio of the density of a gas to the density of air at the same temperature. A gas with a vapor density greater than 1.0 is heavier than air and will sink in the atmosphere. See also density.
vapor pressure: A measure of a substance's tendency to evaporate; liquids with higher vapor pressures evaporate faster.
viscosity: A measure of a fluid's resistance to flow and spreading. Viscosity decreases as temperature increases. The viscosity of an oil influences cleanup strategies because it affects whether the oil can penetrate into a substrate, and whether it can be readily pumped or skimmed. An oil's viscosity increases as it weathers and loses its more volatile components. Petroleum products range in viscosity by orders of magnitude. Tar, for example, is far more viscous than diesel fuel.
viscous: Thick, resistant to flow, having a high viscosity.
VOC: Volatile Organic Compound. Hydrocarbon compounds that exist in the air and contribute to the formation of smog and/or may themselves be toxic. VOCs often have an odor. Some examples include gasoline, alcohol, and the solvents used in paints. VOCs evaporate quickly and can cause nerve damage and behavioral abnormalities in mammals when inhaled.
volatile: A solid or liquid is volatile if it readily evaporates or vaporizes. Petroleum products have a wide range in volatility; for example, gasoline volatilizes so readily that it can pose a short-term risk of fire or explosion hazard. On the other hand, heavy crude oil contains few volatile hydrocarbons and is much less volatile.
volatility: Readiness to evaporate. The volatility of a given substance increases with temperature.

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