|Definition - A language-change process where — to make it easier to pronounce a word — similar consonantal sounds are made less similar.|
Example - The Latin turtur became the more-easily pronounced turtle in English.
Oxford English Dictionary - The first citation for the word in this sense is from 1874:
"Dissimilation … by which two identical sounds are made unlike, or two similar sounds are made to diverge."
(Sweet, English Sounds, 13)