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.

Examples include:

  • 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.