From a presumed derivative of optanomai; visuality, i.e. (concretely) an apparition -- vision.
see GREEK optanomai
1. the act of exhibiting oneself to view: ὀπτασιαι κυρίου, 2 Corinthians 12:1 (A. V. visions; cf. Meyer at the passage) (ἐν ἡμέραις ὀπτασίας μου, Additions to Esther 4:1, 44 (13) 2. a sight, a vision, an appearance presented to one whether asleep or awake: οὐρανίῳ ὀπτασία, Acts 26:19; ἑωρακέναι ὀπτασίαν, Luke 1:22; with the genitive of apposition ἀγγέλων, Luke 24:23. A later form for ὄψις (cf. Winers Grammar, 24), Anthol. 6, 210, 6; for מַרְאֶה, (Theod.) Daniel 9:23; Daniel 10:1, 7f.