Collocation refers to the appearance or occurrence of groups or pairs of words, particularly when more frequent than random chance would suggest.
Wikipedia defines collocation this way: In corpus linguistics, a collocation is a sequence of words or terms that co-occur more often than would be expected by chance.
Chance does not mean pure randomness or arbitrariness; in this sense refers to the preference for certain words being more commonly paired than other choices which have the same meaning.
- Crystal clear
- Forbidden fruit
- Middle Management
In these cases, for example 'crystal' + 'clear' has become an established phrase.
There is a statistical definition of collocation, see the Wikipedia article for more information.