ἐπόπτης, ἐπόπτου, ὁ (from unused ἐπόπτω);

1. an overseer, inspector, see ἐπίσκοπος; (Aeschylus, Pindar, others; of God, in 2Ma 3:39 2Ma 7:35; 3Ma 2:21; Additions to Est 5:1; ἀνθρωπίνων ἔργων, Clement of Rome, 1 Cor. 59, 3 [ET]).

2. a spectator, eye-witness of anything: so in 2Pe 1:16; inasmuch as those were called ἐπόπται by the Greeks who had attained to the third (i. e. the highest) grade of the Eleusinian mysteries (Plutarch, Alcib. 22, and elsewhere), the word seems to be used here to designate those privileged to be present at the heavenly spectacle of the transfiguration of Christ.