The Collaborative International Dictionary of English v.0.48:

Sinusoid \Si"nus*oid\, n. [Sinus + -oid.] (Geom.)
   The curve whose ordinates are proportional to the sines of
   the abscissas, the equation of the curve being y = a sin x.
   It is also called the curve of sines.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sinusoid
    n 1: tiny endothelium-lined passages for blood in the tissue of
         an organ
    2: the curve of y=sin x [syn: sine curve, sinusoid]