There are many definitions of collocation. We think of collocations in the same way as expressed by Benson et al.: "In any language, certain words combine with certain other words or grammatical constructions. These recurrent, semi-fixed combinations, or collocations, can be divided into two groups: grammatical collocations and lexical collocations."
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Reference: Benson, M. Benson, E., Ilsen, R.F. (1986). The BBI combinatory dictionary of English: A guide to word combinations. Amsterdam/Philadelphia: John Benjamins.