Funny story. The original term "DOTA" was an acronym, for "Defense of the Ancients", which was the original Warcraft 3 mod of which the later game, "Dota 2", was based. Since the original DOTA was a community mod using assets from a commercial (IE, trademarked) Blizzard property, it was in sort of a gray area in regards to re-commercialization of what was essentially the same gameplay and characters. To avoid further standing for Blizzard to reclaim its trademarked properties, Valve registered the trademark "Dota" specifically as a word that doesn't refer to anything, officially eliminating the acronym from the name and somehow distancing the new engine from trouble with the trademark.