trepan UK /tɹɪˈpæn/US /tɹəˈpæn/noun 2verb 2forms 5 noun2 •A tool used to bore through rock when sinking shafts. •A surgical instrument used to remove a circular section of bone from the skull; a trephine. verb2 •To create a large hole by making a narrow groove outlining the shape of the hole and then removing the plug of material remaining by less expensive means. •To use a trepan; to trephine. formstrepaned · trepanned · trepaning · trepanning · trepans