Short for Rule Eleven of the [United States] Federal Rules of Civil Procedure, which provides that in signing a pleading (especially a complaint or defense) a party’s lawyer is certifying that the pleading (or case) is:
(a) Not being filed for an improper purpose;
(b) that the claims, defenses, and other legal contentions in the pleading have a reasonable basis in law or can be justified by a “non-frivolous argument” for a new legal principle or extension of existing law to be made;
(c) that the factual assertions either have evidentiary support or will likely have such support after discovery; or
(d) that denials of facts (as in a defense) are justified.
Violations of Rule 11 can result in sanctions being imposed on a party’s lawyer(s) up to and including the counterparty’s attorneys’ fees. Rule 11 motions should be filed with care. It is not unusual for an ill-judged Rule 11 motion to itself attract Rule 11 sanctions.