The Collaborative International Dictionary of English v.0.48:
Ruga \Ru"ga\, n.; pl. Rugae. [L.] (Nat. Hist.) A wrinkle; a fold; as, the rugae of the stomach. [1913 Webster]WordNet (r) 3.0 (2006):
ruga n 1: (anatomy) a fold or wrinkle or crease; "rugae of the stomach"