mucusPronunciation: /ˈmjuːkəs/Translate mucus | into German | into Italian | into Spanish Definition of mucus noun [mass noun] a slimy substance, typically not miscible with water, secreted by the mucous membranes and glands of animals for lubrication, protection, etc.. a gummy substance found in plants; mucilage. Origin: mid 17th century: from Latinmucus in other Oxford dictionaries Definition of mucus in the US English dictionary