The Phoneme Theory

The phoneme is a minimal abstract language unit realized in speech in the form of speech sounds opposable to other phonemes of the same language to distinguish the meaning of morphemes and words. (by Щерба)Phoneme. Series of phoneme.
The definitions of the phoneme vary greatly.
L.V.Shcherba: the phoneme may be viewed as a functional, material and abstract unit.
V.A.Vassilyev: The phoneme is a smallest unit capable of distinguishing one word from another word, one grammatical form of word from another
Bloch: phoneme is a class of phonemically similar sounds contrasting and mutually exclusive with all similar classes in the language
Jacobson: phoneme is a minimal sound by which meaning may be discriminated

In this definition three aspects of the phoneme are represented:

1. material

2. abstract

3. functional