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)

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