A movie cliche
that if it didn't exist it would have to be invented
- Beloved character catches an incurable rare and exotic disease
- Beloved character becomes increasingly more beautiful/brave/intelligent as disease progresses
- Beloved character kicks bucket
So named after actress Ali MacGraw, who played the female lead in 1970's Love Story (more recent examples not cited for fear of spoilers). The term was apparently coined by Roger Ebert.
Naturally Love Story didn't invent this --TV Guide mentions Bette Davis's Dark Victory as an early example and there's always the old Victorian Novel Disease and The Lady of the Camelias1. It did however present what was thought to be a hoary contrivance in modern terms, refurbishing it for the worldly baby boomers.
1Thanks to, uhm, TheLady