As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:
Obligate aerobe, an organism that cannot survive without oxygenObligate anaerobe, an organism that cannot survive in the presence of oxygenObligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphereObligate biped, Bipedalism designed to walk on two legsObligate carnivore, an organism dependent for survival on a diet of animal flesh.Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cellObligate parasite, a parasite that cannot reproduce without exploiting a suitable hostObligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functionsObligate symbionts, organisms that can only live together in a symbiosis