As an adjective, obligate means "by necessity" and is used mainly in biology in phrases such as: Obligate aerobe, an organism that cannot survive without oxygen Obligate anaerobe, an organism that cannot survive in the presence of oxygen Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere Obligate biped, Bipedalism designed to walk on two legs Obligate carnivore, an organism dependent for survival on a diet of animal flesh. Wikipedia