Idhn
The authorisation to transmit: the permission given by the guide to pass on the dhikr, the pact or spiritual direction.
Idhn means ‘permission’, ‘authorisation’. In a Sufi path it is the technical word that settles every question of authenticity: no one guides because he decides to, but because he has been authorised by the one who held that charge. Without idhn, the most learned of men is not a murshid.
The authorisation has degrees, detailed in the literature of the path: authorisation to transmit the dhikr and lead the sessions (the case of the muqaddem), authorisation to give the pact, full authorisation to direct the path, which makes the recipient the successor. It can be explicit and written, or declared before witnesses; in the Qadiriya Boutchichiya it is recorded in the spiritual testaments each guide leaves to his community.
That is why the first question to ask anyone presenting himself as a master is: from whom do you hold this charge? See the full criteria in finding a Sufi master in France.