"Lineage, in the Tibetan context, means a body of teachings which comes down through a particular succession of accomplished Lamas. Lineage has a visionary origin and there would generally (but not always) be a set of texts which describe the practices which belong to the lineage. There will be a series of transmissions, a series of visionary practices and a body of teaching which would have been handed down through various masters to their students. That is what constitutes a lineage."
Ah, but I was so much older then, Ngak’chang Rinpoche, Aro Encyclopaedia