rhotacism
noun
rho·ta·cism ˈrō-tə-ˌsiz-əm 
1
: a defective pronunciation of r
especially : substitution of some other sound for that of r
2
linguistics : the historical change of a voiced consonant sound (such as the alveolar consonants \\z\\, \\d\\, \\l\\ or \\\) to an r-like consonant
Word History
Etymology
borrowed from German Rhotacismus, from Middle Greek rhōtakízein "to use the letter ρ or the sound r excessively or incorrectly" + German -ismus -ism — more at rhotacize
First Known Use
1830, in the meaning defined at sense 2
Medical Definition
rhotacism
noun
rho·ta·cism ˈrōt-ə-ˌsiz-əm
: a defective pronunciation of r
especially : substitution of some other sound for that of r