Dyuloka is a Sanskrit term for "heavenly world". It appears in the Vedic text Shatapatha Brahmana, in verses 16.6.1.8–9 as well later texts.Dyuloka, Monier Monier-Williams, English Sanskrit Dictionary with Etymology, Oxford University Press, page 500 Its root is Dyu (द्यु) which in the Rigveda means "heaven, shining, sky".Dyu, Monier Monier-Williams, English Sanskrit Dictionary with Etymology, Oxford University Press, page 4996.51.5, Rigveda, Wikisource
The term appears in the Upanishads, where it connotes "sky or heaven", as in sun lighting it up. For example, in the commentary to the Yajnavalkya-Gargi dialogue of section 6.2 in the Brihadaranyaka Upanishad, Radhakrishnan translates Dyuloka as heaven.Brihadaranyaka Upanishad, S. Radhakrishnan, George Allen & Unwin, verse VI.2.9, page 312
In another context, Dyuloka is the realm of existence (samsara) where souls are reborn as gods and goddesses, to live out a life based on one's karma before they die again, according to the Devi-Bhagavata Purana.
See also Vaikuntha Deva (Hinduism)
Notes
References Derret, Duncan and Smith, Daniel, "The Adyar Library bulletin, Volume 30", 1966, Adyar Library, google books