Secrets
Also said as Hidden areas, Secret rooms, Easter eggs (when they are a joke)
Things placed off the main route for people who look for them: a wall that is not a wall, a ledge you must think to jump to. Their value is in being found, which needs a hint fair enough to notice.
They reward curiosity, and they make a player trust that looking closely is worthwhile, so they look closely at everything. They also give people something to tell each other about.
Say it
“Put one secret in each level, always behind a wall with a slightly off-colour texture. Show a count at the end of the level, so people start looking for that texture.”
Where you’ve seen it
Doom counts secrets on the tally screen after every level, so you know you missed one and go back to press on walls. Wolfenstein 3D's push walls got players pressing on walls before that.
Not to be confused with easter egg
A secret is part of the game, usually with a reward in it. An Easter egg is a joke or a signature, hidden for the fun of it, that gives you nothing to use.