Obligate

• A verb (see obligation)
• An adjective meaning "by necessity" (antonym facultative) and used mostly in biology, especially in the phrases:
• Obligate parasite, a parasite that cannot survive without its host
• Obligate intracellular parasite, a parasitic microorganism that cannot reproduce outside their host cell
• 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 complex organism which obtains required oxygen directly from the atmosphere through respiration rather than extracting dissolved oxygen from water, as, for example, the South American lungfish

See also

• (antonym) Facultative