Epoptēs means an eyewitness — one who has personally observed something firsthand. It appears only once in the New Testament: 2 Peter 1:16, where Peter insists that the apostolic proclamation of Christ's power and coming was not 'cleverly invented myths' but was based on what they saw as epoptai — eyewitnesses of His majesty on the Mount of Transfiguration. The word was also used in Greek mystery religions for the highest level of initiates who had 'seen' the mysteries directly.
Peter's use of epoptēs in 2 Peter 1:16 is a carefully chosen apologetic term. Against those who might dismiss the gospel as mythological speculation, Peter asserts the epistemological foundation of apostolic testimony: direct, personal, eyewitness observation. We did not imagine the Transfiguration — we were epoptai. This emphasis on eyewitness testimony is foundational in the New Testament (Lk 1:2; Jn 19:35; 21:24; 1 Jn 1:1-3) and insists that Christian faith is grounded not in religious feeling but in historical encounter with a real Person.