In phonetics, the term that describes the smallest unit of sound that can distinguish meaning is a phoneme.
A phoneme is an abstract unit of sound that can change the meaning of a word.
Example: The words "pat" /pæt/ and "bat" /bæt/ differ in only one sound (/p/ vs. /b/), meaning /p/ and /b/ are distinct phonemes in English.
Phonemes are language-specific; some sounds that are phonemic in one language may not be in another.