A hangover that is caused by the body being conditioned to consumption of
alcohol, and triggered by memory/repetition.
If every Friday you get
drunk, and have a hangover every
Saturday as a result, your body becomes conditioned to feeling hungover upon waking up every Saturday.
If one friday you do not drink, but wake up feeling hungover anyway, due to your body just plain being used to the routine, you are the victim of a phantom hangover.
***Not an official medical condition, but it happens.