One commenter says "the definition of insanity was Einstein ... who else has it been attributed to???" Well, it is often attributed to Einstein, and also to Mark Twain and Ben Franklin, but so far there's no proof it existed before about 1980.
In the excellent Yale Book of Quotations (2006) -- a scholarly collection, not just a roundup of favorite alleged sayings -- editor Fred R. Shapiro found the earliest statement of the sentiment in Rita Mae Brown's "Sudden Death," published in 1983: "Insanity is doing the same thing over and over again, but expecting different results."
And just last month, at the Answers.com entry (which also cites Brown), commenter Davidt 9 offered a slight antedating:
The quote "Insanity is repeating the same mistakes and expecting different results" is contained in the Basic Text of Narcotics Anonymous which was published in 1982. The review form of the book was distributed to members in 1981 and work on the book began in 1979. All of this predates Rita Mae Brown's book.He gave a link to the book, which does, as advertised, have the relevant quote on page 11 (25th page of the PDF). I also found a 1980 pamphlet from the Hazelden Foundation, "Step Two: The Promise of Hope," which quotes the same aphorism, so perhaps it got its start in the literature of addiction and recovery.
And of course there may not be an original author; probably these three sources picked up a formulation that had been percolating in the spoken language, possibly in less eloquent variations, just as it was settling into the pithy form we now consider good enough for Einstein.
*Unsuccessfully, to judge from the comments and e-mail.