rho·ta·cism 
(rōtə-sĭz′əm)
(rōtə-sĭz′əm)n.
1. The change of a sound such as (s) or (z) to (r) in the history of a language, such as the change of Proto-Indo-European intervocalic s to r in Latin, resulting in alternations such as that seen between the singular noun genus, "kind, sort" and its plural, genera. Also called rhoticism.
2.
a. The inability to articulate the (r) sound as a rhotic consonant.
b. The substitution of the (r) sound with another sound, such as (w), because of this inability.
3. The articulation of a sound other than (r) as (r), especially the articulation of (z) as (r).
[German Rhotacismus, from Byzantine Greek rhōtakismos, excessive or erroneous use of the letter rho : Greek rhō, the letter rho; see RHO + Greek -takizein in iōtakizein, to pronounce the letters eta and upsilon with the sound of iota (iōta, iota + -izein, verb suff., with inserted -k- from attikizein, to speak Attic Greek, and soloikizein, to speak substandard Greek, make a solecism).]