Water nymphs of the Rigveda, wives of the Gandharvas. Vasishtha is said to be born of an Apsaras.
Sources: Vedic Mythology, A. A. Macdonell (1897); Hymns of the Rigveda, tr. R. T. H. Griffith (1896), hymns read for the Vedic layer · see Apsarases's web