Horologion

[1] It provides the acolouthia (ἀκολουθίαι, akolouthíai), the fixed portions of the Divine Service used every day at certain canonical hours.

Horologium is the latinized version of the Greek hōrológion (ὡρολόγιον), from hṓra (ὥρα, "time period, hour"), lógos (λόγος, "writing, recording") + -ion (-ιον), together originally meaning a sundial, clepsydra, or other timekeeping device.

In Byzantine Greek, the word was repurposed to also denote the Eastern books of hours, records of the hymns and prayers to be offered at the proper times of each day.

Several varieties of horologia exist, the most complete of which is the Great Horologion or Horologium (Ancient Greek: Ὡρολόγιον τò μέγα, Hōrológion tò méga; Church Slavonic: Великий Часословъ, Velikij Chasoslov; Romanian: Ceaslovul Mare, Orologhionul Mare).

Great Horologia also contain a list of the saints commemorated throughout the year with their troparia and kontakia; selected propers for Sundays; moveable feasts from the Menaion, Triodion, and Pentecostarion; and the various canons and other devotional services.