Asuras (Sk.). Exoterically, elementals and evil, gods—considered maleficent; demons, and no gods. But esoterically—the reverse. For in the most ancient portions of the Rig Veda, the term is used for the Supreme Spirit, and therefore the Asuras are spiritual and divine It is only in the last book of the Rig Veda, its latest part, and in the Atharva Veda, and the Brâhmanas, that the epithet, which had been given to Agni, the greatest Vedic Deity, to Indra and Varuna, has come to signify the reverse of gods. Asu means breath, and it is with his breath that Prajâpati (Brahmâ) creates the Asuras. When ritualism and dogma got the better of the Wisdom religion, the initial letter a was adopted as a negative prefix, and the term ended by signifying “not a god”, and Sura only a deity. But in the Vedas the Suras have ever been connected with Surya, the sun, and regarded as inferior deities, devas.
Source: H.P.Blavatsky - The Theosophical Glossary