A species was a group of living organisms capable of interbreeding. The galaxy and its many planets were teeming with diverse species, with members of a species sharing a fixed set of observable biological characteristics. Some species were sentient, capable of intelligent thinking, while others were not,[source?] with more than twenty million sentient species in the galaxy.
Species whose body plan resembled that of humans were referred to as humanoid.[source?] According to Lugubrious Mote, most sentient species in the galaxy were about the same size, ranging in height between about half a meter to three meters.