Phraseological units, or idioms represent what can probably be described as the most picturesque, colourful and expressive part of the languages vocabulary.
Metaphors. The ambiguousness of these interesting word groups may lead to an amusing misunderstanding, especially for children. Puns are frequently based on the ambiguousness of idioms. Together with synonymy and antonymy, phraseology represents expressive resources of vocabulary. Used with care is an important warning because speech overloaded with idioms loses its freshness and originality. Idioms, after all, are ready-made speech units, and their continual repetition sometimes wears them out: they lose their colours and become trite cliches. Such idioms can hardly be said to ornament or enrich the language. Oral or written speech lacking idioms loses much in expressiveness, colour and emotional force. There are some other terms denoting more or less the same linguistic phenomenon: set-expressions, set-phrases, phrases, fixed word-groups, collocations. Free word-groups are so called not because of any absolute freedom in using them but simply because they are each time built up anew in the speech process whereas idioms are used as ready-made units with fixed and constant structures. There are two major criteria for distinguishing between phraseological units and free word-groups: semantic and structural.
The semantic shift affecting phraseological units does not consist in a mere change of meanings of each separate constituent part of the unit. The meanings of the constituents merge to produce an entirely new meaning: The structural criterion brings forth pronounced distinctive features characterising phraseological units and contrasting them to free word-groups.
