Investigations of English phraseology began not long ago. Difference in terminology («set-phrases», «idioms» and «word-equivalents») reflects certain differences in the main criteria used to distinguish types of phraseological units and free word-groups. The term «set phrase» implies that the basic criterion of differentiation is stability of the lexical components and grammatical structure of word-groups.
The term «idioms» generally implies that the essential feature of the linguistic units under consideration is idiomaticity or lack of motivation. Chafe lists four features of idioms that make them anomalies in language: non-compositionality, transformational defectiveness, ungrammaticality and frequency asymmetry.
According to the theory of prof. A.V. Kunin phraseological units as distinguished from free word-groups have three main parameters:
1. Phraseological units are language units, their characteristic feature is semantic complexity, i.e. full and partial transference of meaning, e.g., to burn one’s fingers is used figuratively, it is a metaphor based on the similarity of action.
2. Structural separability and semantic cohesion, e.g., to kick the bucket – to die, Mrs. Grundy, Tom, Dick, and Harry(перший-ліпший)
3. A phraseological unit is never formed on a generative pattern of a free word-combination, one cannot predict the formation of a phraseological units. The patterns in phraseology are of some other character; they are patterns of description (unpredictable). There are some grammatical patterns (noun phrases, verbal phrases), some semantic patterns (metaphoric formation, metonymic formation).
In conclusion we can say that phraseological units are habitually defined as non-motivated word-groups that cannot be freely made up in speech but are reproduced as ready-made units; the other essential feature of phraseological units is stability of the lexical components and grammatical structure.