Any material which is a gas at 20°C (68°F) or less and 101.3 kPa (14.7 psi) of pressure and which is ignitable at 101.3 (14.7 psi) when in a mixture of 13% or less by volume with air; or has a flammable range at 101.3 kPa (14.7 psi) with air of at least 12% regardless of lower limit.

Note: Labels used to identify a subsidiary risk must have the class number obliterated.

Also see:

What are the Federal Regulations for Hazardous Materials Labeling?