The Collaborative International Dictionary of English v.0.48:

Peritrochium \Per`i*tro"chi*um\, n. [NL., fr. Gr. ?; peri`
   around + ? a wheel.] (Mech.)
   The wheel which, together with the axle, forms the axis in
   peritrochio, which see under Axis.
   [1913 Webster]