We all read about Mahasiddhas but what really is a mahasiddha. I found a rather interesting and compelling definition on Wikipedia:-
Mahasiddha (maha meaning "great" and siddha meaning "adept") is a term for someone who embodies and cultivates the "siddhi of perfection." They are a certain type of yogin/yogini recognized in Vajrayana Buddhism. Mahasiddhas were tantric practitioners, or tantrikas who had sufficient empowerments and teachings to act as a guru or tantric master. A siddha is an individual who, through the practice of sadhana, attains the realization of siddhis, psychic and spiritual abilities and powers. Their historical influence throughout the Indic and Himalayan region was vast and they reached mythic proportions which is codified in their songs of realization and hagiographies, or namthar, many of which have been preserved in the Tibetan Buddhist canon. The Mahasiddhas are the founders of Vajrayana traditions and lineages, such as Dzogchen and Mahamudra.
Robert Thurman explains the symbiotic relationship between Tantric Buddhist communities and the Buddhist universities such as Nalanda which flourished at the same time:
The Tantric communities of India in the latter half of the first Common Era millennium (and perhaps even earlier) were something like “Institutes of Advanced Studies” in relation to the great Buddhist monastic “Universities.” They were research centers for highly cultivated, successfully graduated experts in various branches of Inner Science (adhyatmavidya), some of whom were still monastics and could move back and forth from university (vidyalaya) to “site” (patha), and many of whom had resigned vows of poverty, celibacy, and so forth, and were living in the classical Indian saiñnyãsin or sãdhu style. I call them the "psychonauts" of the tradition, in parallel with our “astronauts,” the materialist scientist-adventurers whom we admire for their courageous explorations of the “outer space” which we consider the matrix of material reality. Inverse astronauts, the psychonauts voyaged deep into “inner space,” encountering and conquering angels and demons in the depths of their subconscious minds.