[oh_sigh]

Just what it sounds like - there are objects in a picture and you need to spot them. You'd get a list of 10 objects(guitar, owl, hamper, etc) and you need to find them in a picture of a room. The objects frequently aren't hidden per se but are just hard to spit because there might be 100 objects in the picture.

[derefr]

At its core, sure, but then imagine taking that and making it the core mechanic of a larger experience, in the way that "moving between tiles and attacking in a direction" is the core of a roguelike.

You can add time pressure; things that only appear once you've found other things; hybridize the mechanics with those of other genres, like adventure-game-like puzzle minigames to combine or use objects; etc.

In some sense, any game where progression is gated by a threshold of gathered collectables—and where the hard part is detecting the collectable, not navigating to or otherwise retrieving the collectable—is a hidden-object game. Pokemon Snap is a hidden-object game—the objects are shot compositions. The Ace Attorney games are hidden-object games—the objects are literal evidence, but then during the trials they're evidence+opportunity pairings. "Hacking" games, where you detect vulnerabilities to exploit, are hidden-object games. Trade-war or Elite type games (i.e. inefficient-market exploitation simulators) are, on an abstract level, hidden-object games. :)

And, of course, games in other genres can employ hidden-object mechanics, too. Platform games have special coins in out-of-the-way locations, where the challenge is just inferring that that was a visitable place to begin with. RPGs have treasure that shows up as little often-hard-to-spot motes of light on the field rather than big obvious chests. Etc.