A siddha is a soul that has shed all karma and left embodiment for ever, the second of the five saluted in the Jain prayer and the final goal of the whole path.
Liberation completed
Where the arihant is omniscient but still embodied, the siddha has cast off the remaining karmas and the body itself at the end of that embodied life, and the pure soul rises to the summit of the universe, there abiding for ever in its innate perfection, infinite knowledge, perception, bliss and energy, wholly free of the cycle of birth and death. The siddha is thus the fully liberated soul, the end state of the entire Jain endeavour, and no further becoming remains to it.
The goal saluted
The siddha is saluted second in the Navkar Mantra, after the arihant, and the order is significant, for the arihant, still teaching in the world, is honoured first as the source of the accessible path, while the siddha is the goal toward which that path leads, and both are saluted before the living offices of the community. The standing is not a role that anyone occupies in the world but the condition of the perfected souls beyond it, so the siddha is less an office than the realised end of the path, and the daily salutation of the siddhas keeps that final goal, the liberation of the soul from all karma and embodiment, before the mind of every Jain who recites the prayer.