From "Ruling by Term"
Reveal
Any revealed cards are shown to all players.
If we turn that phase over, we
could infer that "any card that is shown to all players is revealed". But maybe that is not correct? It sounds weird to me that a card that is shown to all players in the game is not considered "revealed", though.
Filtering all cards with "reveal" in the text box, it seems that "reveal" is used in different ways:
a) Showing opponet cards from your hand, playdeck or discard pile is "revealing" (An Untimely Whisper, Aware of their Ways, Crebain...)
b) Turning face up a site to move a company there is "revealing" (minion Himring, minion Tolfalas...)
c) Turning face up an on-guard hazard and executing its mechanics is "revealing" (Foolish Words, Searching Eye...)
d) Bringing your "avatar" into play is "revealing" it (The Lidless Eye, Sacrifice of Form...)
e) Maybe other uses of "revealing"? I didn't find any more, but maybe...
The Great Hunt and Pallando's ability use "discard face up" but not "reveal", probably because it's kind of weird to reveal each card to your opponent ("I discard Risky Blow", "I discard Ambusher"...) instead of just discarding them and leaving that info totally to your opponent's awareness during the game. But it's a total guess.