SB 6.9.18

yenāvṛtā ime lokās
 tapasā tvāṣṭra-mūrtinā
sa vai vṛtra iti proktaḥ
 pāpaḥ parama-dāruṇaḥ
Word for word: 
yena — by whom; āvṛtāḥ — covered; ime — all these; lokāḥ — planets; tapasā — by the austerity; tvāṣṭra-mūrtinā — in the form of the son of Tvaṣṭā; saḥ — he; vai — indeed; vṛtraḥ — Vṛtra; iti — thus; proktaḥ — called; pāpaḥ — personified sin; parama-dāruṇaḥ — very fearful.
That very fearful demon, who was actually the son of Tvaṣṭā, covered all the planetary systems by dint of austerity. Therefore he was named Vṛtra, or one who covers everything.

In the Vedas it is said, sa imāḻ lokān āvṛṇot tad vṛtrasya vṛtratvam: because the demon covered all the planetary systems, his name was Vṛtrāsura.