The Collaborative International Dictionary of English v.0.48:

Planimeter \Pla*nim"e*ter\, n. [Plani- + -meter. Cf.
   Planometer.]
   An instrument for measuring the area of any plane figure,
   however irregular, by passing a tracer around the bounding
   line; a platometer.
   [1913 Webster] Planimetric

WordNet (r) 3.0 (2006):

planimeter
    n 1: a measuring instrument for measuring the area of an
         irregular plane figure [syn: integrator, planimeter]