Which term describes the phonetic process where sounds lose similarity to their neighboring sounds?

The correct term for the phonetic process where sounds lose similarity to their neighboring sounds is dissimilation. This phenomenon occurs when two identical or similar sounds are made less alike in terms of their articulation, thereby reducing any potential auditory ambiguity or difficulty in pronunciation. An example of dissimilation can be seen in some dialects where the word "fifth" is pronounced as "fift," reducing the similarity between the two 'f' sounds.

In contrast, the concept of assimilation refers to the process where sounds become more similar to nearby sounds. Vowel reduction involves the process by which vowels in unstressed positions may be pronounced less distinctly, and hardening generally refers to a different phonetic change involving the strengthening of a consonant sound, which isn't applicable to the concept of sounds losing similarity. Therefore, dissimilation is the appropriate term to describe the indicated phonetic process.