The Collaborative International Dictionary
Ultra vires \Ul`tra vi"res\, [Law Latin, from L. prep. ultra beyond + vires, pl. of. vis strength.] Beyond power; transcending authority; -- a phrase used frequently in relation to acts or enactments by corporations in excess of their chartered or statutory rights.
Douglas Harper's Etymology Dictionary
Latin, literally "beyond powers," from ultra (see ultra-) + vires "strength, force, vigor, power," plural of vis. Usually "beyond the legal or constitutional power of a court, etc."
Wiktionary
phr. (context legal English) beyond the legal power or authority of a person or body
WordNet
adj. beyond the legal power or authority of a person or official or body etc; "an ultra vires contract" [ant: intra vires]
adv. beyond the scope or in excess of legal power or authority
Wikipedia
Ultra vires is a Latin phrase meaning "beyond the powers". If an act requires legal authority and it is done with such authority, it is characterised in law as intra vires ("within the powers"). If it is done without such authority, it is ultra vires. Acts that are intra vires may equivalently be termed "valid" and those that are ultra vires "invalid".