The Collaborative International Dictionary of English v.0.48:

Rhotacism \Rho"ta*cism\, n. [Gr. "rwtaki`zein to use the letter
   r ([rho]) overmuch: cf. F. rhotacisme.]
   An oversounding, or a misuse, of the letter r; specifically
   (Phylol.), the tendency, exhibited in the Indo-European
   languages, to change s to r, as wese to were.
   [1913 Webster]