Within the area of corpus linguistics, collocation is defined as a pair of words (the 'node' and the 'collocate') which co-occur more often than would be expected by chance.

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 'weird' if collocational preferences are violated. This makes collocation an interesting area for language teaching.

Corpus Linguists specify a Key Word in Context (KWIC) and identify the words immediately surrounding them. This gives an idea of the way words are used.

The processing of collocations involves a number of parameters, the most important of which is the significance function, 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 significance scores are simply used to rank the results. Commonly used significance functions include mutual information, t-score , log-likelihood .

Some examples for collocates of 'bank' are: central, river, account, manager, into, 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.

[Note: the (previous) entry below does not strictly speaking apply to collocations, but rather to idioms]

Within the area of corpus linguistics, a collocation can be defined as a sequence of two or more consecutive words that has characteristics of a syntactic and semantic unit, and whose meaning cannot be derived compositionally, i.e. it cannot be derived directly from the meaning of its components.

Common features of collocations are:

• Non-compositionality: The meaning of a collocation is not a straightforward composisition of the meaning of its parts. For example, the meaning of kick the bucket has nothing to do with kicking buckets. (Kick the bucket means to die.)
• Non-substitutability: We cannot substitute a word in a collocation with a related word. For example, we cannot say yellow wine instead of white wine although both yellow and white are the names of colors.
• Non-modifiability: We cannot modify a collocation or apply syntactic transformations. For example, John kicked the green bucket or the bucket was kicked has nothing to do with dying.