One who imagines the 'worst-case scenario' as something so completely horrific it borders on absurd. A 'catastrophist' usually starts by saying "Yeah but what if..." then, talking at a normal speed, increases speed until they're talking very quickly as they relate their vision of the 'worst-case scenario' which may involve any (or all) of the following things: A psycho with an axe, getting chopped up into little pieces, ending up homeless and destitute, sucking dick for crack, any number of serious diseases or medical afflictions (See Chuck Palahniuk's "Choke" the main character had catastrophist tendancies)
see also "final destinationism" (yet to be defined)
"Dude, it's one stupid term-paper"
"Yeah but if I don't do well on this one then my GPA falls and I'll never get into a good college and I won't get a good job and that will DEVASTATE my mother and she'll commit suicide and I'll have to take care of my little brother and he'll get Lupus, need a kidney and I will be forced to sell my ass on the street because I can't afford the surgery because I don't have health-care all because I failed this stupid paper!"
"Stop being such a catastrophist, it's not even worth 10% of your grade!!!"