Paramanu is a Sanskrit word used in the Vaisheshika school of thought in the Indian Philosophy to denote that part of a bhuta, which is indivisible, indestructible and eternal.
In other words, the nitya (eternal) form of the four dravyas prithvi, jal, tejas and vayu is called as Paramanu.
[4] Thus the literal meaning of the compound Sanskrit word paramanu is ultimate atom.
They are prithvi, jal, tejas, vayu, aakasha, kaal, dika, atman and manas.
[1] Paramanu is that smallest part of these bhutas which is indivisible, indestructible and whose existence is eternal.