octane number

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

• noun A numerical representation of the antiknock properties of motor fuel, compared with a standard reference fuel, such as isooctane, which has an octane number of 100.

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

• noun a measure of the antiknock properties of an automobile fuel; isooctane having an octane number of 100, a fuel with the same antiknock properties as a mixture of 90% isooctane and 10% heptane having an octane number of 90

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a measure of the antiknock properties of gasoline