A solecism is a phrase that transgresses the rules of grammar. The term is often used in the context of linguistic prescription; it also occurs descriptively in the context of a lack of idiomaticness.

The word originally was used by the Greeks for what they perceived as grammatical mistakes in their language. Ancient Athenians considered the dialect of the inhabitants of Soli, Cilicia to be a corrupted form of their pure Attic dialect and labelled the errors in the form as "solecisms" (Greek: σολοικισμοί, soloikismoí; sing.: σολοικισμός, soloikismós). Therefore, when referring to similar grammatical mistakes heard in the speech of Athenians, they described them as "solecisms" and that term has been adopted as a label for grammatical mistakes in any language; in Greek there is often a distinction in the relevant terms in that a mistake in semantics (i.e., a use of words with other-than-appropriate meaning or a neologism constructed through application of generative rules by an outsider) is called a barbarism (βαρβαρισμός barbarismos), whereas solecism refers to mistakes in syntax, in the construction of sentences.