Cohabitation (cohab) is housing more than one reptile together — which for the vast majority of pet reptiles is discouraged, because it causes stress, competition, and hidden bullying.
Most beginner reptiles are solitary and do best alone; tegus and monitors in particular must live alone. Signs of a bad cohab (one animal not eating, hiding, or losing weight) are easy to miss until harm is done. When unsure, house separately.