In corpus linguistics, a collocation is a sequence of words or terms that co-occur more often than would be expected by chance. In phraseology, collocation is a sub-type of phraseme. An example of a phraseological collocation, as propounded by Michael Halliday, is the expression strong tea. While the same meaning could be conveyed by the roughly equivalent *powerful tea, this expression is considered incorrect by English speakers. Conversely, the corresponding expression for computer, powerful computers is preferred over *strong computers. Phraseological collocations should not be confused with idioms, where meaning is derived, whereas collocations are mostly compositional.
Expanded definition 
Collocations are partly or fully fixed expressions that become established through repeated context-dependent use. Such terms as 'crystal clear', 'middle management', 'nuclear family', and 'cosmetic surgery' are examples of collocated pairs of words.
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:
- ‘Free Combination’ ↔ ‘Bound Collocation’ ↔ ‘Frozen Idiom’
In dictionaries 
As long ago as 1933, Harold Palmer's Second Interim Report on English Collocations highlighted the importance of collocation as a key to producing natural-sounding language, for anyone learning a foreign language. Thus from the 1940s onwards, information about recurrent word combinations became a standard feature of monolingual learner's dictionaries. As these dictionaries became 'less word-centred and more phrase-centred', more attention was paid to collocation. This trend was supported, from the beginning of the 21st century, by the availability of large text corpora and intelligent corpus-querying software, making possible a more systematic account of collocation in dictionaries. Using these tools, dictionaries such as the Macmillan English Dictionary and the Longman Dictionary of Contemporary English included boxes or panels with lists of frequent collocations.
There are also a number of specialized dictionaries devoted to describing the frequent collocations in a language. These include (for Spanish) Redes: Diccionario combinatorio del español contemporaneo (2004), and (for English) the LTP Dictionary of Selected Collocations (1997) and the Macmillan Collocations Dictionary (2010).
See also 
- Agreement (linguistics)
- Collocational restriction
- Collostructional analysis
- Compound noun, adjective and verb
- Government (linguistics)
- Lexical item
- Phrasal verb
- Siamese twins (English language)
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