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Ap (áp-) is the Vedic Sanskrit term for "water", which in Classical Sanskrit only occurs in the plural āpas (sometimes re-analysed as a thematic singular, āpa-), whence Hindi āp. The term is from PIE hxap "water".[note 1] The Indo-Iranian word also survives as the Persian word for water, āb, e.g. in Punjab (from panj-āb "five waters"). In archaic ablauting contractions, the laryngeal of the PIE root remains visible in Vedic Sanskrit, e.g. pratīpa- "against the current", from *proti-hxp-o-. In Tamil, Appu (Tamil form of "Ap") means water, and has references in poetry.
In the Rigveda, several hymns are dedicated to "the waters" (āpas): 7.49, 10.9, 10.30, 10.137. In the oldest of these, 7.49, the waters are connected with the drought of Indra. Agni, the god of fire, has a close association with water and is often referred to as Apām Napāt "offspring of the waters". In Vedic astrology, the female deity Apah is the presiding deity of the Purva Ashadha asterism, meaning "first of the aṣāḍhā", with aṣāḍhā "the invincible one" being the name of the greater constellation.
In Hindu philosophy, the term refers to water as an element, one of the Panchamahabhuta, or "five great elements". In Hinduism, it is also the name of the deva Varuna a personification of water, one of the Vasus in most later Puranic lists.