A combination of words in a language, that happens very often and more frequently than would happen by chance.
For example, blonde + hair is a collocation. However, yellow + hair is not a collocation. Another example of a collocation is show + increase, e.g. The chart shows an increase in the number of students applying for university last year. However, present + increase is not a collocation and is therefore not accurate vocabulary usage.
A collocation dictionary allows you to search the different collocations that a word has.