What defines a "phoneme" in linguistics?

A "phoneme" is defined as the smallest distinctive unit of sound in a language that can differentiate meaning between words. For example, changing the initial sound in the words "bat" and "cat" changes the meaning of the words, making /b/ and /k/ phonemes. Phonemes do not carry meaning by themselves; however, when combined, they form words that have specific meanings.

The focus on phonemes is crucial for understanding how sound patterns in language work and how these sounds can lead to variations in meaning. This concept is foundational in phonetics and phonology, illustrating the complexity of language at its most fundamental level. The other options describe different aspects of language: graphic representations relate to writing, units of meaning pertain to semantics, and segments of written language refer to orthography, which do not exclusively address the nature of phonemes.