Final answer:
A phoneme is the smallest unit of sound in a language that changes meaning. Different combinations of phonemes form morphemes, which convey meaning. For example, the word 'cat' has three phonemes.
Step-by-step explanation:
The smallest unit of sound in a language that changes meaning is called a phoneme. Phonemes are basic sound units of a language and are combined to form morphemes, which are the smallest units of language that convey meaning. For example, the word 'cat' has three phonemes: /k/, /æ/, and /t/. Changing any of these phonemes would alter the meaning of the word.