Second level object
A second level object (or second level noun) is one that is only mentioned in the description of another object (as opposed to in room descriptions or similar). A third level object is mentioned in the description of a second level object, and so on.
>look You are in a narrow hallway. There is a picture hanging on the wall. >x picture The picture is of a ship being tossed around in a stormy, blue-green sea. The sails are billowing wildly. >x sails (second level) The sails are almost being torn away from the ship. There is a strange symbol on the main sail. >x symbol (third level) It looks like an eagle in flight, grasping a snake in its claws.
It can be confusing for players if the depth of object descriptions is inconsistent within a game; on the other hand, deeply describing an object can be one way to hint to players that it is significant.