The Collaborative International Dictionary of English v.0.48:

Isoperimetrical \I`so*per`i*met"ric*al\, a. [Gr. ?; ? equal + ?
   circumference: cf. F. isop['e]rim[`e]tre. See Perimeter.]
   (Geom.)
   Having equal perimeters of circumferences; as,
   isoperimetrical figures or bodies.
   [1913 Webster]