Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

decubitus
     n : a reclining position (as in a bed)