Aamir Khan suffers from the illness of short term memory loss in Ghajini. His character is supposed to have a maximum memory span of 15 minutes as a result of an injury he suffers on his head.
We did some research on the disease and found that the technical term for this loss of memory is known as 'Anterograde amnesia'. In simpler terms, it stands for loss of memory of what happens after the event that caused the amnesia. It is different from 'Retrograde amnesia' where memories prior to the event are forgotten. What is scary though is that till date, anterograde amnesia remains a mysterious ailment and cure for that is yet to be established.
In Ghajini, Aamir faces from exactly the same illness as he does not remember anything that happened more than 15 minutes back, even though he frequently has flashbacks of Asin's murder. It's the effects of this disease that he is still able to recollect some events prior to the injury but cannot recall day–to–day information or new facts that were presented to them after the injury occurred.
In the past there have been number of movies where a major protagonist is shown to be suffering from this illness. While Memento is the inspiration for Ghajini, Drew Barrymore's character in the film 50 First Dates too suffered from the same illness. Same was the case for Dory in Finding Nemo, Scott Bakula's character Jonathan Archer in the Star Trek: Enterprise episode "Twilight", and Chihiro Shindo from EF: A Fairy Tale Of The Two.