1. Loosely, communication, as in ‘the expression of opinions’.
2. Any word or phrase that is a unit of meaning. ‘Mind your Ps and Qs’ is an English expression.
3.How something is communicated as distinct from what is communicated; often referred to as form rather than content. Hjelmslev, referring specifically to language, argued that expression (similarly distinguished from content) consisted of both form (language, formal syntactic structure, and style) and material substance (phonemes or graphemes).