Plain English Meaning
A blip on the radar screen that the controller can see but hasn't yet connected to a known aircraft. The position is showing up, but the system isn't telling the controller who it is or what it's doing.
Full Definition
A radar return displayed on a controller's scope that has not been associated with a specific aircraft identity, flight plan, or transponder data block. The target appears as a position symbol only, without the linked information (callsign, altitude, groundspeed) that accompanies a tracked target.
Context Anchor
Seen in air traffic control discussions, especially when comparing identified aircraft with other radar returns near them.