Phoneme is a sound unit. Phonemes are the smallest sound units in a language. Language, taken as a phonemic system, is studied by identifying the most basic units of sound (phonemes) and the many rules that govern the combinations of these units.
In the twentieth century, this way of studying the system of language has had a huge impact on the study of other sign systems, such as human culture, myths, and narratives.
A phoneme is a sound with a distinctive function in a system of aural signs (for example, spoken English).
In this sense, /h/ in hat is obviously different from /m/ in mat.
This is a difference for English speakers, while the difference between the way you and I pronounce /h/ (even in cases where it is very noticeable) can be ignored.
Sound acquires the status of a phoneme in language because it is a difference that shows differences, something that allows us to distinguish something from another (this word from another, this meaning from another).
From the point of view of structural linguistics, any language that is approached as a system of aural signs is not just an accumulation of signs that function as phonemes due to the presence of internal qualities in them, it is a real system whose units (in this case phonemes) are such because of their oppositions (differences) from other units in the system, and not because of their internal qualities.
As this structuralist approach has proven its fruitfulness in linguistics (the study of language), it could be demonstrated in other topics. One such topic is, for example, the perception of communicative behavior.
The first step of the structuralist approach to communicative behavior must be to identify the most basic units within the system of this behavior.
Pike suggests that these units be called actemes.
Related: Articulation
