The Collaborative International Dictionary of English v.0.48:

Rheostat \Rhe"o*stat\ (r[=e]"[-o]*st[a^]t), n. [Gr. "rei^n +
   stato`s standing still.] (Elec.)
   A contrivance for adjusting or regulating the strength of
   electrical currents, operating usually by the intercalation
   of resistance which can be varied at will. --Wheatstone.
   --Rhe`o*stat"ic, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rheostat
    n 1: resistor for regulating current [syn: rheostat, variable
         resistor]