Phraseological units are (according to Prof. Kunin A.V.) stable word-groups with partially or fully transferred meanings ("to kick the bucket", “Greek gift”, “drink till all's blue”, “drunk as a fiddler (drunk as a lord, as a boiled owl)”, “as mad as a hatter (as a march hare)”).
3 types of lexical combinability of words:
1). Free combination
Grammatical properties of words are the main factor of their combinability.
Ex.: I’m talking to you. You are writing.
Free combinations permit substitution of any of its elements without semantic change of the other element.
2). Collocations.
Ex.: to commit a murder
(Bread & butter, Dark night, Blue sky, Bright day)
They are the habitual associations of a word in a language with other particular words. Speakers become accustomed to such collocations.
Very often they are related to the referential & situational meaning of words.
Sometimes there are collocations, which are removed from the reference to extra-linguistic reality.
(collocations involving, colour words)
Ex.: to be green with jealousy
-Red revolution
3). Idioms
Idioms are also collocations, because they consist of several words that tend to be used together, but the difference – we can’t guess the meaning of the whole idiom from the meanings of its parts.
This criterion is called the degree of semantic isolation.
In different types of idioms – it is different.
Ex.: to cry a blue murder = to complain loudly