In classical music the curse of the ninth is a fear of the 9th Symphony among composers. The origin of the curse stems from the surprising number of major composers who died after completing nine symphonies, or even in the course of writing their ninth. Beethoven, Schubert, Dvořák and Vaughan Williams fall in the former category; Bruckner in the latter.
The curse has affected a number of noted composers. Gustav Mahler was so afraid of it that he did not call his ninth symphonic work a symphony, rather labeling it Das Lied von der Erde. He finished this work unscathed, only to die while working on his tenth.
Alexander Glazunov stopped work on his ninth in 1910 supposedly due to fear of the curse and never wrote another symphony. He managed to live another 26 years.
The curse is also said to have played a role in the musical paralysis that beset Sibelius later in his life, and which led to the destruction of his 8th Symphony.
The first really notable composer after Beethoven to complete more than nine symphonies was Dmitri Shostakovich, who lived decades after completing his ninth and wrote six more symphonies in that time. Less well known composers had broken the "curse" before him, however - the 19th century composer Joachim Raff, quite well known in his day, wrote eleven symphonies, for example.
The curse still holds weight with a number of modern composers. Malcolm Arnold has no intention on writing another after his ninth. Roger Sessions also never wrote more than nine.
Some symphonies that are now listed as 9th symphonies were originally published under different numbers and have been renumbered by musicologists in recent years. This has nothing to do with the curse of the ninth but simply the order in which they were published rather than the order in which they were written. For example, Schubert's Symphony in C "The Great" was originally published as his 7th, although it postdated his unfinished 8th symphony. Dvořák's Symphony in E minor "From the New World" was originally published as his 5th.