Sports term for the most competitive/difficult group of a pool play tournament. Used frequently in soccer (football, to non-Americans).

While in theory, groups should be about even in strength, there's quite often one group that's significantly better than others.

For example, in World Cup 2002, Group F is widely considered to be the "Group of Death". The group includes Argentina, England, Sweden, and Nigeria. It features no pushovers and any of the 4 teams are capable of advancing far in the tournament. However, they'll knock each other out (with 2 of the 4 advancing).
It's arguable that any team in Group F could beat any team in a few other weaker groups.

Since drawings for pool play tournaments are usually random or semi-random, supporters of teams in a Group of Death usually murmur about some sort of conspiracy against their squad, while claiming favoritism for teams who had better fortune in their placement.