| Dissimilation is a linguistic phenomenon in which, of two similar sounds nearby, one is replaced by a less similar one. This puts more phonetic distinctness between them, making them more salient, but goes against the usual phonetic processes of assimilation and simplification.
The clearest example is a Latin adjective ending. This is -alis unless the word contains an L, in which case it dissimilates to -aris. These become modern English -al, -ar. Thus cauda 'tail', nasum 'nose' give us caudal, nasal, but compare polar, familiar. If the root contains both L and R, the ending is dissimilar from the last of them, as in lateral.
The rule applied actively in Latin, but doesn't in English; we merely inherited the dissimilated words. In the case of familiar, as this has shifted semantically and no longer means 'of the family', a new word familial has been coined, disregarding the rule.
The word pilgrim comes ultimately from Latin per-agr- 'through field', via peregrinus then with dissimilation of the first L.
It is not a regular process in English, but the non-standard pronunciation chimley exhibits it: M and N are both nasal, L is not. The words diphthong, diphtheria have two fricatives and sometimes the first dissimilates, becoming a plosive: dipthong, diptheria.
The East African language Kikuyu is pronounced Gikuyu. This is an example of voicing dissimilation, which occurs in various Bantu languages in the area. | 
