Rheum (/ r uː m /; from Greek: ῥεῦμα rheuma 'a flowing, rheum') is a thin mucus naturally discharged from the eyes, nose, or mouth, often during sleep (contrast with mucopurulent discharge). [ 1 ] [ 2 ] [ 3 ] Rheum dries and gathers as a crust in the corners of the eyes or the mouth, on the eyelids, or under the nose. [ 3 ]