A total party kill, almost always abbreviated to simply TPK, is roleplaying gamer slang for a event which results in the deaths of the entire adventuring party. TPKs are more common in some systems than others, due to the varying levels of lethality that abound in roleplaying games. In a typical 3rd or 4th edition Dungeons & Dragons game, for example, a TPK usually occurs because the players were too dumb or stubborn to back down from a powerful threat which they were meant to run away from, but in a game like Dark Heresy the threat of a TPK is ever present even in minor skirmishes. In games such as Call of Cthulhu, some campaigns seem to be designed to cause TPKs, leading to a strange influx of American PIs to whatever Mythos-ridden hellhole the party is unlucky enough to be exploring.
Excessive TPK is usually the mark of a bad GM, who will view it as the gold standard of a "successful" campaign. Although a "rocks fall, everyone dies" scenario could technically be a TPK, they are not usually described as such.
Note that when the party in question is the total population of a planet, Imperial experts have created the term "exterminatus" to accurately describe the phenomenon. If playing a Dungeons & Dragons derivative, the best way to accomplish this is the infamous Deck of Many Things. Give it to your players, sit back and watch even the ones who should know better destroy themselves without having to lift a finger to do it yourself.