a totally liberated soul.
Siddhas are the liberated souls. They are no longer among us because they have completely ended the cycle of birth and death. They have reached the ultimate highest state, salvation. They do not have any karmas, and they do not collect any new karmas.
This state of true freedom is called Moksha.
Siddhas are experiencing unobstructed bliss (eternal happiness). They have complete knowledge and perception and infinite power. They are formless and have no passions and therefore are free from all temptations.