Collocation comprises the restrictions on how words can be used together, for example which prepositions are used with particular verbs, or which verbs and nouns are used together. Collocations are examples of lexical units. Collocations should not be confused with idioms.
Collocations can be in a syntactic relation (such as verb-object: 'make' and 'decision'), lexical relation (such as antonymy), or they can be in no linguistically defined relation. Knowledge of collocations is vital for the competent use of a language: a grammatically correct sentence will stand out as 'awkward' if collocational preferences are violated. This makes collocation an interesting area for language teaching.
The processing of collocations involves a number of parameters, the most important of which is the measure of association, which evaluates whether the co-occurrence is purely by chance or statistically significant. Due to the non-random nature of language, most collocations are classed as significant, and the association scores are simply used to rank the results. Commonly used measures of association include mutual information, t scores, and log-likelihood.
Rather than select a single definition, Gledhill proposes that collocation involves at least three different perspectives: (i) cooccurrence, a statistical view, which sees collocation as the recurrent appearance in a text of a node and its collocates, (ii) construction, which sees collocation either as a correlation between a lexeme and a lexical-grammatical pattern, or as a relation between a base and its collocative partners and (iii) expression, a pragmatic view of collocation as a conventional unit of expression, regardless of form. It should be pointed out here that these different perspectives contrast with the usual way of presenting collocation in phraseological studies. Traditionally speaking, collocation is explained in terms of all three perspectives at once, in a continuum:
Collocates of 'bank' are: central, river, account, manager, merchant, money, deposits, lending, society. These examples reflect a number of common expressions, 'central bank', 'bank or building society', and so forth. It is easy to see how the meaning of 'bank' is partly expressed through the choice of collocates.
High collocates with probability, but not with chance: a high probability but a good chance
Herd collocates with cows, but not with sheep: a herd of cows but a flock of sheep