Presence, coming, arrival; a technical term for a royal visit or official arrival.
Used 24 times in the NT, 17 for Christ's return. In secular Greek, parousia was the technical term for the ceremonial arrival of a king — public, visible, unmistakable. Paul uses it in 1 Thess 4:15 where the Lord descends with a shout, voice, and trumpet. Matthew 24:27 compares it to lightning flashing across the entire sky. The word itself argues against any "secret" coming — a royal parousia is by definition public.
Appearing, manifestation, brightness; a visible, glorious display.
Used 6 times in the NT for Christ's return. Literally "a shining upon" — emphasizing VISIBILITY. In 2 Thess 2:8, the lawless one is destroyed by "the brightness/epiphaneia of his coming/parousia" — two visibility words stacked. Titus 2:13 calls it "the glorious appearing." Incompatible with a secret event.