fossa UK /ˈfɒsə/US /ˈfɑsə/noun 2/ˈfuː.sə/ noun 1forms 3 noun2 •A pit, groove, cavity, or depression. •A long, narrow, shallow depression on the body of an extraterrestrial body, such as a planet or moon. fossa /ˈfuː.sə/ noun1 •A large nocturnal reddish-brown catlike mammal (Cryptoprocta ferox) of the civet family, endemic to the rainforests of Madagascar. It is slender, long-tailed and has retractile claws and anal scent glands. formsfossae · fossae · fossas