In Diksha Initiation a Sat-Guru (true god-man) gives you some of his God Consciousness, in regular Initiation any human teacher can give you a meditation technique.
The difference is in Diksha Initiation the initiate must *qualify* for the experience by his past spiritual sadhana (practice), and be able to sustain the tremendous spiritual power brought on by the Guru, obviously only a true realized Master can do this and an advanced disciple. In ordinary Initiation any human teacher (even a book) can instruct one in techniques to empower the sincere seeker! I would submit TM is such a technique, perhaps one of the more effective techniques available but still not a diksha Initiation. It may however, lead to Diksha Initiation based on merit like any reputable meditation technique. Sources: Swami Yoganada, The Second Coming. "The disciple,in turn, must be advanced and deserving in order to be able to receive such a 'baptism' in Omniscience (diksha) by his advanced guru who is one with CC." Kirpal Singh, The Crown of Life. "But when the student..succeeds in rising above physical consciousness, he finds the Radiant Form of his Master waiting unsought to receive him. Indeed, it is at this point that the real Guru-disciple or teacher-student relationship is established. Up to this stage, the Guru had been little more than a human teacher...."
