(philately) a philatelically inspired letter flown on the first flight of a new airline service.

(euphemistic, biblical) To lower one’s garment, especially to urinate or defecate.

(anatomy, zoology) An operculum; a series of bones found in bony fish that serves as a facial support structure and a protective covering for the gills.

plural of cover artist

Of or pertaining to a covenant.

(espionage) A cover which is a position at a governmental agency (e.g. posing as a member of the diplomatic service).

(espionage) The assumption of a covert role in an organization which does not have ties to the government of the operative in question.

(topology) A cover whose members are all open sets.

plural of cover crop

An instrument for locating rebars and measuring the depth of concrete that covers them.