1. Good physical condition; being in shape or in condition; SYN. physical fitness, good shape, good condition.
2. The condition of being suitable; SYN. fittingness.
3. The quality of being qualified.
In genetic theory, a measure of the success with which a genetically determined character can spread in future generations. By convention, the normal character is assigned a fitness of one, and variants (determined by other alleles) are then assigned fitness values relative to this. Those with fitness greater than one will spread more rapidly and will ultimately replace the normal allele; those with fitness less than one will gradually die out.