Introduction to Tantra Sastra - Aghori

SIDDHI
SIDDHI is produced by sādhana. The former term,
which literally means “success,” includes accomplishment,
achievement, success, and fruition of all kinds. A
person may thus gain siddhi in speech, siddhi in
mantra, etc. A person is siddhi also who has perfected
his spiritual development. The various powers attainble—namely,
aṇ imā, ahima, garimā, prapti, prākāmyā,
īśitva, vaśtva—the powers of becoming small, great,
light, heavy, attaining what one wills, and the like—are
known as the eight siddhis. The thirty-ninth chapter of
the Brahmavaivarta Purāṇ a mentions eighteen kinds,
but there are many others including such minor accomplishments
as nakhadarpaṇ asiddhi or “nail-gazing.”
The great siddhi is spiritual perfection. Even the
mighty powers of the “eight siddhis” are known as the
“lesser siddhi,” since the greatest of all siddhis is full
liberation (mahānirvāṇ a) from the bonds of phenomenal
life and union with the Paramātmā, which is the
supreme object (paramārtha) to be attained through
human birth.