In Hinduism, a siddha is "one who is accomplished". It refers to perfected masters who have achieved a high degree of physical as well as spiritual perfection or enlightenment. Siddha may also refer to one who has attained a siddhi, paranormal capabilities.
Siddhas may broadly refer to siddhars, naths, ascetics, sadhus, or yogis because they all practice sādhanā.
In Jainism, siddhas are the liberated souls who have destroyed all karmas and have obtained moksha. Siddhas do not have a body; they are soul in its purest form. They reside in the Siddhashila, which is situated at the top of the Universe.
Example via www.ramdass.org: Bhagavan Nityananda