I found this term along with the terms "internal examiner" and "committee of examiners" in University websites, referring to the academic examining body for Master's and Ph.D. theses. Here is an explanation of both internal/external examiners:

University of Nottingham:

Role of the External Examiner
The External Examiner must read the student's thesis and should lead the viva voce examination. The External Examiner must in particular come to a view as to whether the student's research work and knowledge are of a standard which are comparable to those of students being examined at other institutions for the same degree.

Role of the Internal Examiner
The Internal Examiner must read the student's thesis and participate in the student's viva voce examination, and come to a view as to whether the student's research work and knowledge meet the standard which would normally be expected of a student in the School submitting for that degree, and the University's criteria for the award of the degree.
In addition s/he should act as the facilitator of the administrative arrangements for the examination of the thesis and of the viva voce examination.
S/he should seek to ensure that, during the viva voce examination, the student is given a fair opportunity to defend his/her work and that the External Examiner is aware of any extenuating circumstances which have a bearing on the student's case.

I believe that given your context, external examiner is the better choice here...