In AD&D 1st edition, what is the difference between a concealed door and a secret door?
They are defined on p97 of the Dungeon Master's Guide
Concealed doors are doors hidden in some way:
- Behind a curtain
- Covered with plaster
- Trap door under a rug
A PC can normally find a concealed door just by checking his surroundings well.
Secret doors are portals that look like a normal wall to the naked eye. Typically it takes more effort to discover a secret door.