There's no one definition. In one Series the Yankees were up 3 runs in a game 7 with 6 outs left and lost the lead tied the game and then lost it. That was brutal. In 1986, game 6 was a brutal loss, but it was not a World Series losing loss, because it was only game 6. 2004 speaks for itself. To be one win away and lose 4 in a row. Well, that's pretty brutal.