Amā (Ama) is one of the rays of the Sun. Amāvasyā (Amavasya) is the tithi on which the moon enters into Sūryamaṇḍala (Suryamandala; the rounded area covered by the says of the Sun) and stays upon the ray called Amā . The moon resides upon the ray called Amā on that day, that is why the name Amāvasyā (Amavasya) —
amākhyaraśmau vasati amāvasyā tataḥ smṛtā.