Also known as an irrevocable commitment or a lock-up. A binding agreement by a target shareholder to accept a takeover offer or vote in favour of a scheme of arrangement. An irrevocable undertaking may be either hard (binding in all circumstances), soft (ceases to be binding if a higher offer is announced) or semi-hard (ceases to be binding if a higher offer is announced which exceeds the existing offer by an agreed amount).

For further information, see Standard document, Deed of irrevocable undertaking: synopsis and Practice note, Takeover Code know-how: Irrevocable commitments.