What is the smallest unit of sound that infants can distinguish before starting to imitate language?

The smallest unit of sound that infants can distinguish is known as a phoneme. Phonemes are the basic building blocks of spoken language, representing distinct sounds that can change meaning in words. For example, the change in the initial sounds of the words "bat" and "cat" demonstrates how different phonemes can create different words entirely. Infants have a remarkable ability to hear and differentiate these sounds even before they begin to speak, which lays the foundation for language acquisition.

Morphemes refer to the smallest units of meaning, and while they are crucial in understanding language, they are not the smallest units of sound. Lexemes are the abstract units of meaning related to a word or its base form, and although they are important in the context of vocabulary, they are above the level of individual sounds. Phonological units can encompass various sound patterns in language but do not specifically refer to the most basic unit common in distinguishing different words. Thus, phoneme is the most precise term to describe the smallest unit of sound perceived by infants prior to their own language imitation.