Octan

Part of speech: adjective, noun

Definitions

1. Describing a form of malarial fever that recurs every other day, counted inclusively as every eighth day in older medical usage
2. Relating to an intermittent malarial fever whose attacks come roughly every second day, a pattern once called octan
3. Used of a malaria-type fever that returns at intervals of about two days, following the older octan classification

Etymology: Octane is a chemist's coinage from the Greek "okto", meaning eight, plus the suffix "-ane", which marks a saturated hydrocarbon. Any molecule of that family with eight carbon atoms takes the name, so it describes a class of compounds rather than one substance. The term became familiar at the petrol pump. Engineers testing engine knock needed a scale, so they used reference fuels: pure heptane, which scored 0, and a branched eight-carbon isomer, isooctane (2,2,4-trimethylpentane), which scored 100. A fuel's octane rating is the percentage of isooctane in a blend that knocks the same way. A high rating therefore means resistance to knock, not a higher number of carbon atoms, though the name has stuck.