Allophonic variation
Allophonic variation refers to the phenomenon where a single phoneme (a unit of sound in a language) can be pronounced in different ways depending on its surrounding sounds. These different pronunciations, called allophones, are not contrastive, meaning they don't change the meaning of a word.
Examples:
The vowel /i/ can be pronounced in a higher position before a fricative consonant, like in “beat” and “feet”, and in a lower position before a velar consonant, like in “bit” and “fit”.
The consonant /l/ can be pronounced as a clear sound at the beginning of a word or after a vowel, like in “lip” and “milk”, and as a dark sound after a consonant, like in “milk” and “talk”.
Peperkamp12, S., Pettinato, M., & Dupoux, E. (2003). Allophonic variation and the acquisition of phoneme categories.
Beňuš, Š., & Beňuš, Š. (2021). Allophonic Variation in English. Investigating Spoken English: A Practical Guide to Phonetics and Phonology Using Praat, 115-136.
Sproat, R., & Fujimura, O. (1993). Allophonic variation in English/l/and its implications for phonetic implementation. Journal of phonetics, 21(3), 291-311.