Doom is a status in Final Fantasy VIII. Instant Death is applied once the counter expires. The effect lasts 16 seconds (default Battle Speed setting), despite the counter starting at 8 (a likely allusion to the game's number in the series), and ends after battle. Zombie will also cancel out Doom's effect if cast on the character afflicted. Doom is functionally similar to the Petrifying status.
Doom will never work on undead enemies or bosses, and various other enemies may also be immune or highly resistant. Enemies' resistance to Doom is a separate stat from their resistance to Death.