What is "cetane number"

n. 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.

Cetane number or CN is an indicator of the combustion speed of diesel fuel. It is an inverse of the similar octane rating for gasoline (petrol). The CN is an important factor in determining the quality of diesel fuel, but not the only one; other measurements of diesel's quality include (but are not limited to) energy content, density, lubricity, cold-flow properties and sulphur content.