Refer to figures.
In a perfect world a direct reading compass (DRC) would always indicate the correct magnetic heading.  Unfortunately aircraft generate their own magnetic field due to ferromagnetic construction  materials or electrical equipment.  The aircraft's magnetic field has an influence on the Earths magnetic field causing the compass indication to deviate from Magnetic North.  The amount of deviation varies with aircaft heading because the interaction of the 2 magnetic fields is dependent on orientation.

The direction indicated on a DRC is known as Compass North therefore deviation is defined as the angular difference between magnetic north and compass north.  Deviation is expressed in degrees East or West or by plus and minus.  To assist the pilot in flying the correct magnetic heading compass deviation, which is determined by a ‘compass swing’, is indicated by a deviation card (see figure) placed in the vicinity of the DRC to show the amount of deviation on various headings.

Is always the same for all heading in the same aircraft → INCORRECT.  Deviation varies with aircraft heading.

Is the angular difference between true north and magnetic north → INCORRECT.  That's the definition of magnetic variation.

Is the angular difference between true north and compass north → INCORRECT.  It' s the angular difference between magnetic north and compass north.

Is the angular difference between magnetic north and compass north → CORRECT.