ὁρίζων · horizōn — LSJ
separating circle (cf. ὁρίζω 1.1b), horizon, Autol. Sph. 5, Ti.Locr. 97a ; ὁτοῦ ὁρίζοντος κύκλος Arist. Mete. 363a27 ; ὁ ὁ. κύκλος Id. Cael. 297b34, al. ; ὁ αἰσθητὸς ὁ., opp. ὁ λόγῳ θεωρητός, Gem. 5.56, 57 ; οἱ ὁρίζοντες Ti.Locr. 97d.
Pythag. name for 9, because it limits, i.e. finishes, the series of units, Theol.Ar. 57.