A Self-Realized Yogi or master of Tantra can also be referred to as a 'Siddha'.
In some cases however the word 'Siddha' was used to mean one whose focus was on the development and display of special 'attainments' or 'powers' ('Siddhis'), instead of being focused on the true purpose of Yoga - 're-Union' with the Supreme Self..
The great teacher Sri Ramana Maharshi said the true 'Siddhi', or attainment, is 'Atma Siddhi' - the power of Self-Knowledge, or 'Self-Realization'.
Understood in this sense, the Maha Siddhas were the great masters of the traditions.
The importance of the Siddhas in the History of Yoga and Tantra:
There are several Yoga traditions that refer to their lineage as originating with one or more Siddhas, or 'Mahasiddhas' (Great Siddhas).
One of these, which I have had closer contact with is the 18 Siddha tradition of Tamil Nadu.
Great Siddhas and great Yogis ('Mahayogis') are associated with the origins of both Yoga and Tantra, in South and North India as well as the origins of Tibetan Buddhist Tantra.
Many Siddhas appear in the lineages of several different traditions.
According to some sources, the great Buddhist teacher Nagarjuna was either the disciple of Matsysendranath (known in the 18 Siddha tradition also as Machamuni or Machindranath) or of Gorakshanath (known also as Gorakkar, or Korakkar).
The traditions of Tantra and Yoga have been intimately linked throughout history in both India and Tibet.
It is only in recent times that the traditions have become entirely distinct - in what could be termed 'Gym Yoga', and 'Bedroom Tantra'.
- Peter Littlejohn Cook