The Collaborative International Dictionary of English v.0.48:

Pedesis \Pe*de"sis\, n. [NL., from Gr. ph`dhsis a leaping.]
   Same as Brownian movement, under Brownian.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pedesis
    n 1: the random motion of small particles suspended in a gas or
         liquid [syn: Brownian movement, Brownian motion,
         pedesis]