a collocation is a sequence of words or terms that co-occur more often than would be expected by chance (from Wikipedia|Collocation) A collocation, e.g. fatal error, typically consists of a node (error) and a collocate (fatal). Collocations can have different strength, e.g. nice house is a weak collocation because both nice and house can combine with lots of other words, on the other hand, the Opera House is a strong collocation because it is very typical for opera to occur next to house and, at the same time, opera does not combine with many other words.