What is "vritra"

In the early Vedic religion, Vritra ( Sanskrit: , , lit. 'enveloper') is a serpent or dragon, the personification of drought and adversary of Indra. In Hinduism, Vritra is identified as an Asura. Vritra was also known in the Vedas as Ahi ( Sanskrit: , lit. 'snake'). He appears as a dragon blocking the course of the rivers and is heroically slain by Indra.