cetane number

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A performance rating for diesel fuels, with a higher number indicating greater fuel efficiency. The number corresponds to the percentage of cetane in a cetane-heptamethylnonane mixture, or (and originally) in a cetane-methylnaphthalene mixture, with the same ignition performance as the fuel being rated.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A performance rating of diesel fuel corresponding to the percentage of cetane in a mixture with isocetane (or, formerly, methylnaphthalene) that would have the same performance.