Centimorgan (cM): A unit of measure of genetic recombination frequency. One cM is equal to a 1% chance that a marker at one genetic locus will be separated from a marker at another locus due to crossing over in a single generation. In humans, 1 cM is equivalent, on average, to 1 million base pairs. The centimorgan is named after the pioneering (and Nobel Prize winning) geneticist Thomas Hunt Morgan.