Lewis & Short

2. bēta, n. indecl. (beta, ae, f., Aus. Technopaegn. c. Litt. Mon. v. 13), = βῆτα, the Greek name of the second letter of the alphabet (pure Lat. be; v. B): hoc discunt omnes ante alpha et beta puellae, * Juv. 14, 209.
Hence, prov., the second in any thing (as alpha is the first), Mart. 5, 26.