The Amber hallucination is a term used to describe House's hallucination of Amber Volakis, James Wilson's girlfriend who died in the Season 4 finale, Wilson's Heart. She began appearing as an hallucination, congratulating House on solving another case.
She eventually haunted him during the last few episodes of Season 5.
At first House was amused but things got very serious when Chase nearly died due to allergic reaction to the type of body butter that a stripper at his bachelor party used, leading House to assume that since Amber was part of his subconscious, he was trying to kill Chase too.
It was also revealed that Amber was not only a hallucination but part of House's subconscious which manifested itself in her form due to the guilt House was experiencing over Kutner and Amber's deaths too. She made a few appearances during House Divided by offering him advice on how to treat a patient.
Unlike the original Amber who had been competitive but had shown human emotion at times, in Under My Skin, the Amber hallucinations who were originally an annoyance soon became more and more sinister, cruel, and violent, even going so far to calmly slice her own arm open with a scalpel.
When House began to try and figure out what was wrong with him, Amber began mocking and tormenting him. She also reminded House that every option would be terrible for him: Schizophrenia meant that he could never work again as a physician, while Multiple Sclerosis or stopping Vicodin would cause him severe pain.
House realized the problem was the Vicodin, and during his withdrawal, Amber constantly tortured him, laughing and telling him he was worthless. However, once the withdrawal period was over and House managed to get some sleep, the hallucination appeared to have vanished for good.
In the season finale Both Sides Now she appeared one last time when House realized he never detoxed at all or slept with Cuddy. She said: "So this is the story you made up about who you are. It's a nice one," to which a hallucination of Kutner also added "Too bad it isn't true." his mind and experiencing Psychosis.
As a result, House checked into Mayfield Psychiatric Hospital and underwent a detox to ensure that the Amber hallucination was finally put to rest.
When on screen, Amber is seen wearing her normal doctor's coat with the Princeton-Plainsboro badge identifying her as a doctor although she is deceased.
Amber's hallucination appears one last time in the series finale, Everybody Dies, along with slew of other characters but unlike many of her previous appearances, she encourages House to live.