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Daena is a feminine noun which translates to "that which is seen or observed". In Zoroastrianism: An Introduction to an Ancient Faith, Peter Clark suggests that the term might also be tied to the Avestan root "deh" or "di-" to gain understanding.
The Avestan language term – trisyllabic daēnā in Gathic Avestan and bisyllabic dēnā in Younger Avestan – continues into Middle Persian as dēn, which preserves the Avestan meanings. For comparison, it is similar to Sanskrit dhénā which means thought, but thought in its higher and spiritual reaches.  
The concept of Daena is mentioned in the Gathas, a series of seventeen hymns supposedly written by Zoroaster. Daena appears both in the Ahunavaiti Gatha and in the Ushtavaiti Gatha, where it is written that Daena is somehow affiliated with the reward that the faithful will receive in the afterlife. However, references to Daena in the Gathas are brief, leaving much ambiguity on its nature.
Later Avestan writings, such as the Vendidad, describe the concept of Daena further. The Vendidad portrays Daena as something of a psychopomp, guiding good and pure souls over the Chinvat Bridge to the House of Song, Zoroastrian paradise, while the wicked are dragged to the House of Lies, a place of punishment. She is described as being finely dressed, and accompanied by dogs.
Maneckji Dhalla writes in Zoroastrian Theology that on the dawn of the fourth day after death "there appears then to the soul its own daena, or religious conscience in the shape of a damsel of unsurpassed beauty, the fairest of the fair in the world."
Daena (din in modern Persian) is the eternal Law, whose order was revealed to humanity through the Mathra-Spenta ("Holy Words"). Daena has been used to mean religion, faith, law, even as a translation for the Hindu and Buddhist term Dharma, often interpreted as "duty" or social order, right conduct, or virtue. The metaphor of the 'path' of Daena is represented in Zoroastrianism by the muslin undershirt Sudra, the 'Good/Holy Path', and the 72-thread Kushti girdle, the "Pathfinder".
- Clark, Peter (1998), Zoroastrianism: An Introduction to an Ancient Faith, 1, Sussex: Sussex Academic Press: 69-70.