Epiphanēs (ἐπιφανής) means manifest, famous, or notable — the adjectival form of epiphaneia (appearing, manifestation). It appears once in Acts 2:20, quoting Joel's prophecy of "the great and notable [epiphanēs] day of the Lord." The cognate epiphaneia (G2015) is used extensively for Christ's appearances.
The "Day of the LORD" as epiphanēs — manifested, visible, unmistakable — is eschatological promise and warning. Joel's oracle (2:31) predicts cosmic signs before "the great and manifested day of the LORD comes." Peter at Pentecost (Acts 2:16-21) applies this to the Spirit's outpouring: the last days have begun, the Day is near. The epiphaneia of Christ is a central NT eschatological concept: His first appearance brought salvation; His second will bring completion. Titus 2:11: "The grace of God has appeared [epiphaneia] that offers salvation." Titus 2:13: "We wait for the blessed hope — the appearing [epiphaneia] of our great God and Savior."
The epiphany tradition in Christian liturgy (January 6) takes its name from epiphaneia — the manifestation of Christ to the Gentiles (Magi). The word carries Hellenistic resonance: divine epiphanies (sudden appearances of gods) were a common Greek religious concept. Paul and the NT writers claim Christ's appearance is the definitive divine epiphany — not a mythic visitation but a historical incarnation.