Expression in words, either speech or writing.
A formal lengthy exposition of some subject, either spoken or written.
An institutionalized way of thinking, a social boundary defining what can be said about a specific topic (after Michel Foucault).
Any rational expression, reason.
To exercise reason; to employ the mind in judging and inferring; to reason.
To engage in discussion or conversation; to converse.
To write or speak formally and at length.
An idiomatic expression in general.
An established expression whose wording is subject to little or no variation, and which may or may not be idiomatic.