Phraseology: what It Is and how It Works
That is what is meant when phraseological units are said to be characterised by semantic unity. In the traditional approach, phraseological units have been defined as word-groups conveying a single concept (whereas in free word-groups each meaningful component stands for a separate concept). 
Phraseology represents expressive resources of vocabulary. In standard spoken and written English today idiom is an established and essential element that, used with care, ornaments and enriches the language. 
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 clichés. Such idioms can hardly be said to "ornament" or "enrich the language".
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