Answer:
Usually the term "residual magnetism" is used in referring to the weak, leftover magnetic force remaining in field coil poles in generators. These generators can be the automotive type, but can also be any other type of generator in which a Direct Current is used in a series of field windings wrapped around field cores to produce magnetic fields around a rotating armature with the purpose of producing electricity from mechanical motion (the rotating armature).
The field winding poles/cores are made of iron, and after the generator has been run a few seconds under load, will retain a weak magnetic field even after the generator has been shut down. This is the "residual magnetism". The majority of older-design generators depend upon this weak, leftover field to restart the voltage produced in the armature, rather than the voltage regulator depending upon a battery to restart the generator field magnetism.
Residual magnetism gets weaker over time; the time depends in part on the quality of the field pole piece and and how strong the current applied to the field coils was at the time the generator was stopped.
To temporarily restore the residual magnetic field, current will need to be applied to the field circuit BRIEFLY; completely disconnecting the generator will be required. Connecting a 6 volt "lantern battery" in the proper polarity across the field winding circuits for a few seconds will establish a residual magnetic field in each field coil/windingzperiodz Proper polarity is critical: backwards polarity will create backwards residual magnetic fields, and the generator will produce backwards-polarity electricity when the armature is spun.