The chapter is organized as follows: section 2 deals with different defi nitions of idiom in Hispanic linguistics. After reviewing them, a new defi nition is proposed. Section 3 explains how the characteristic of fi xedness is defi ned in relation to the form and meaning of idioms. Furthermore, this can also be applied to their structure. Section 4 deals with the types of idioms that correspond to lexical units: nominal, adjectival, verbal and adverbial idioms. In each group, their morphological and syntactic properties, that is, their grammatical characteristics are presented, as well as their semantic ones. The chapter concludes with a consideration regarding the status of idioms in relation to lexical units and other types of lexical combinations.

2 Defi nition of idiom Idioms are the object of study of phraseology, a discipline that, for some researchers, 2 should be concerned with paremies, formulae and collocations, so that all of these would constitute phraseological units of a language. Other authors, 3 however, restrict phraseology to the analysis of idioms, a position that implies that paremies are studied by paremiology, formulae by pragmatics, that collocations are not phraseological units and that only idioms, the central core of phraseological units, should be analyzed by phraseology.