The morphological system of language reveals its properties through the morphemic structure of words. It follows from this that morphology as part of grammatical theory faces the two segmental units: the morpheme and the word.

It is very difficult to give a rigorous and at the same time universal definition to the word, i.e. such a definition as would un­ambiguously apply to all the different word-units of the lexicon. This difficulty is explained by the fact that the word is an extremely complex and many-sided phenomenon. Within the framework of dif­ferent linguistic trends and theories the word is defined as the minimal potential sentence, the minimal free linguistic form, the el­ementary component of the sentence, the articulate sound-symbol, the grammatically arranged combination of sound with meaning, the meaningfully integral and immediately identifiable lingual unit, the uninterrupted string of morphemes.

It is not easy to identify the word because the words are heterogeneous from the point of view of both content and form. To find the criteria of word identification linguists resort to the notions of functional correlation and continuum. Functional correlation connects the elements which have similar and different properties. In fact, within a complex system of interrelated elements there exist two types of phenomena – “polar” and “intermediary”. Polar phenomena stand to one another in an explicit opposition. Intermediary phenomena are located in the system in between the polar phenomena, making up a gradation of transitions. A total of these transitions makes up a continuum. Thus, between proper nouns and common nouns – polar phenomena – there exist different transitions of semi-proper nouns which make up a continuum. Giving a definition to the word on these lines, it is necessary to describe the notional one-stem word and the grammatical morpheme as the opposing polar phenomena. The continuum existing between them is constituted by functional words. Functional words are very limited in number and perform various grammatical functions. In distinction to these, notional words are infinite in number and are nominative units proper.

In this connection, the notional one-stem word and the mor­pheme should be described as the opposing polar phenomena among the meaningful segments of language; these elements can be defined by their formal and functional features most precisely and unambiguously. As for functional words, they occupy intermediary positions between these poles, and their very intermediary status is gradational. In particular, the variability of their status is expressed in the fact that some of them can be used in an isolated response po­sition (for instance, words of affirmation and negation, interrogative words, demonstrative words, etc.), while others cannot (such as prepositions or conjunctions).

Thus, the word is the nominative unit of language built up by morphemes and indivisible into smaller segments as regards its nominative function. In traditional grammar the study of the morphemic structure of the word is based upon two criteria.

The properties of the morpheme and the word are fundamental from the point of view of their systemic status and therefore require detailed investigations and descriptions. The morpheme is a meaningful  segmental component of the word; the morpheme is formed by phonemes; as a meaningful component of the word it is elementary (i.e. indivisible into smaller segments as regards its significative function). The word is a nominative unit of language; it is formed by morphemes; it enters the lexicon of language as its elementary component (i.e. a component indivisible into smaller segments as regards its nominative function); together with other nominative units the word is used for the formation of the sentence - a unit of informa­tion in the communication process.