Plain English Meaning
The number of degrees, measured clockwise from straight off the nose of your airplane, to the radio station the ADF is pointing at. If the needle points straight up, the station is dead ahead (000°). If it points to the right wing, the station is at 090° relative to you.
Full Definition
The angle, measured clockwise from the nose of the aircraft to the line pointing directly at a non-directional beacon (NDB), as displayed by the Automatic Direction Finder (ADF) needle. It is expressed in degrees from 000° (straight ahead) through 360°, and is referenced to the aircraft's longitudinal axis, not to magnetic or true north.