Ten thousand times ten thousand (μυρίαδες μυρίαδων)
Lit., ten thousands of ten thousands. Compare Psa 68:17; Dan 8:10. Μυριάς, whence the English myriad, means the number ten thousand. So, literally, Act 19:19, ἀργυρίου μυριάδας πέντε fifty-thousand pieces of silver; lit., five ten-thousands. In the plural used for an unlimited number. See Luk 12:1; Act 21:20; Heb 12:22; Jud 1:14.
Thousands (χιλιάδες)
Χιλιάς, a collective term like, μυριάς, meaning the number one thousand, is almost invariably used with men in Revelation. See Rev 7:4; Rev 11:13. Only once with a material object (Rev 21:16). With inferior objects χίλιοι a thousand is used. See Rev 11:3; Rev 12:6. These words are the theme of Alford's noble hymn -