The Collaborative International Dictionary of English v.0.48:

Rectus \Rec"tus\ (-t?s), n.; pl. Recti (-t?). [NL., fr. L.
   regere to keep straight.] (Anat.)
   A straight muscle; as, the recti of the eye.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rectus
    n 1: any of various straight muscles