How many idioms are there in English?

The word idiom and its derivatives idiomatic, idiomatically and idiomaticity are used with a wide range of meanings. Idiomatic English is used to refer to fluent language use that sounds like that of a native speaker. Items loosely classed as idioms include colloquial expressions, collocations, acceptable but unusual expressions, and opaque multi-word units. If linguists are pressed to define what an idiom is, they usually say that an idiom is a multi-word unit where the meaning of the whole unit is not clear from the meaning of its parts. The purpose of this paper is to report on a study where one particular use of the term idiom was very carefully defined and to show what the effects of the application of this careful definition were on coming up with a definitive list of idioms. It is argued that carefully distinguishing idioms from other multiword units makes sense for the teaching and leaming of multi-word units because different approaches are needed for the different types of multi-word units. Phrasal verbs were not included in the study.