As previously discussed, an "ultra" is a peak with 1500m of prominence. There is no minimum height, so an ultra can be a forested bump on an island, as long as it has 1500m of prominence. The original definition was 5000 feet,
Various peak list enthusiasts worldwide have compiled a list of ultras for the whole world. According to Wikipedia in 2012, there are 1524 such peaks worldwide. Below are some links to various lists from Bivouac.