Ultimate Wave is one of Cell's ki attacks in his perfect form. He creates a vast, green energy sphere that, upon contacting with the ground, produces a powerful shockwave that travels in both directions from where it hits, inflicting great damage.
Cell used this attack to destroy the Cell Games Arena after he found it too small to fight in. The explosion sent shockwaves across the area.
Appearences in the video games
In Xenoverse, it is depicted as a green electrical ki blast, though is performed in much the same fashion.