Conventional linguistic units which are not predictable simply by knowing the grammar 'rules' and the vocabulary of a language. For this reason, idiomatic expressions are described as 'noncompositional' and have to be learned whole. Idiomatic expressions were studied by Charles Fillmore and Paul Kay in the development of construction grammar (1).

With their colleague Catherine O'Connor they developed a typology of idiomatic expressions. These include: decoding idioms, encoding idioms, grammatical idioms, extragrammatical idioms, substantive idioms, formal idioms, idioms with pragmatic point and idioms without pragmatic point.