46 CFR §188.10-21

This term includes any material or mixture having in the container an absolute pressure exceeding 40 p.s.i. at 70 °F.; or regardless of the pressure at 70 °F., having an absolute pressure exceeding 104 p.s.i. at 130 °F.; or any liquid flammable material having a vapor pressure exceeding 40 p.s.i. absolute at 100 °F. as determined by the Reid method covered by the American Society for Testing Materials Method of Test for Vapor Pressure of Petroleum Products (D-323). Compressed gases are discussed in more detail in 49 CFR parts 171-179.