Mandala-brahmana Upanishad of Shukla-Yajurveda, Chapter IV

Then Yājñavalkya addressed the Puruṣa in the sphere (of the sun) thus: "Pray explain to me in detail the nature of the fivefold division of ākāś." He replied: "There are five (viz.): ākāś, parākāś, mahākāś, sūryākāś, and paramākāś. That which is of the nature of darkness, both in and out is the first ākāś. That which has the fire of the deluge, both in and out is truly mahākāś. That which has the brightness of the sun, both in and out is sūryākāś. That brightness which is indescribable, all-pervading and of the nature of unrivalled bliss is paramākāś. By cognising these according to this description, one becomes of their nature. He is a yogin only in name, who does not cognise well the nine chakras, the six ādhāras, the three lakṣyas and the five ākāś. Thus ends the fourth Brāhmaṇa."