The Collaborative International Dictionary of English v.0.48:
Decubitus \De*cu"bi*tus\, n. [NL., fr. L. de- + cubare, to lie down: cf. F. d['e]cubitus.] (Med.) An attitude assumed in lying down; as, the dorsal decubitus. [1913 Webster] DeculassementWordNet (r) 3.0 (2006):
decubitus n 1: a reclining position (as in a bed)