Generally, the most powerful black magic spell in the popular Final Fantasy series. It always has certain properties in each game:
Despite this spell's awesomeness, it is only practical to use against battles involving more than one enemy. Otherwise, flare is a much better (and cheaper) choice (Warning: flare can be reflected!!). For the most part however, ultima will do at least 5000 damage, and it's very possible for it to do 9999 if your character is strong enough.
Although ultima is a pervasive spell in the Final Fantasy series, it actually started its debut in Final Fantasy 6, and is replaced by doomsday in Final Fantasy 9. It comes back in Final Fantasy 10, however.
The predecessor for ultima is probably meteo, and luminaire looks suspicously similar to ultima in Chrono Trigger. Although I have heard of no evidence concerning this spell's relation to the Ultima Series, one must wonder...