Parliaments and legislative bodies around the world impose certain rules and standards during debates. Tradition has evolved that there are words or phrases that are deemed inappropriate for use in the legislature whilst it is in session. In a Westminster system, this is called unparliamentary language and there are similar rules in other kinds of legislative systems. This includes, but is not limited to, the suggestion of dishonesty or the use of profanity. Most unacceptable is any insinuation that another member is dishonourable. So, for example, in the British House of Commons any direct reference to a member as lying is unacceptable. A conventional alternative, when necessary, is to complain of a "terminological inexactitude".

Exactly what constitutes unparliamentary language is generally left to the discretion of the Speaker of the House. Part of the speaker's job is to enforce the assembly's debating rules, one of which is that members may not use "unparliamentary" language. That is, their words must not offend the dignity of the assembly. In addition, legislators in some places are protected from prosecution and civil actions by parliamentary immunity which generally stipulates that they cannot be sued or otherwise prosecuted for anything spoken in the legislature. Consequently, they are expected to avoid using words or phrases that might be seen as abusing that immunity.

Like other rules that have changed with the times, speakers' rulings on unparliamentary language reflect the tastes of the period. The Table, the annual journal of the Society of Clerks-at-the-Table in Commonwealth Parliaments, includes a list of expressions ruled unparliamentary that year in the national and regional assemblies of its members.