Purushottama is the supreme spirit in Hinduism. Purushottama is a compound word of purusha and uttama (supreme). Purush means pure conscious atma i.e., God or the atma. The earliest use of the word Purusha also occurs in the Rig Veda (Rig Veda X Sukta 90).
The word, in the present context, means the conscious principle in both the animate and inanimate worlds.
The Vedanta philosophy, especially Absolute Monism, propounds the Brahman, the supreme spirit, as omnipresent, omnipotent and omniscient. This envelops everything in this universe, great or small, movable or immovable, sentient or non-sentient.
The Bhagavad Gita describes the whole universe as consisting of the Kshara Purusha (perishable creatures) and the Akshara Purusha (imperishable spirit) XV 16. Both these are known as Kshetra (field0, i.e.,the body and the Kshetrajna (knower of the field, i.e, the atma. (Bhagavad Gita XIII 1).
The one Absolute and Supreme Spirit above the many atmas and material nature is Purushottama. He envelops all the three worlds (universe) and is called the Universal Atma and sustains the same as the immutable Lord (Bhagavad Gita XV 17).
“As I surpass the destructible spirit and as I am the Supreme above the imperishable spirit, I am celebrated as Purushottama in the Vedic literature as well as in the mortal world. On undeluded, who knows me as Purushottama, realizes everything and worships me wholeheartedly (Bhagavad Gita XV. 18-19)
All created things are a mutable nature (Kshara Bhava) VIII.4) and unmanifest is called imperishable (VIII.20).
Thus, according to the Bhagavad Gita, all manifest and gross objects in the universe are of a perishable nature, while the spirit in the jivatma (individual atma) is immutable. The universal atma, which envelops and sustains them both, is the Supreme, Absolute Spirit (purushottama).
Notes taken from Encyclopedia of Hinduism Volume VIII page 343