Variation

Variation in biology usually refers to the heritable differences within a species. This is important in genetics, and the study of evolution in natural populations. Heritable variation is ultimately caused by mutations of genes and chromosomes, and shuffled by recombination. Outcrossing keeps, and inbreeding reduces, heritable variation.
Other variations are caused during life, and are not passed on to the next generation by heredity.
Variation is also a term used in biodiversity for the variety of life forms in a given ecosystem.