set phrase 
noun
An unvarying phrase having a specific meaning, such as ‘raining cats and dogs’, or being the only context in which a word appears, for example ‘amends’ in ‘make amends’.The form Scotch survives, however, in compounds and set phrases....
- Chinese schooling emphasizes the preservation of the oral culture by requiring children to memorize set phrases and to think in mnemonic patterns.
- From the evidence, it seems that the trend towards using people instead of persons is accelerating and that it may not be so long before persons vanishes from the language except in certain set phrases.