Uber Units are a group of units that class as the best unit of a certain race. Each race has at least one, and usually require all buildings and research, as well as a Relic. They are the most expensive units, and for good reason; they are extremely powerful, and some come with special abilities. They usually exceed 10,000 health points, and you will only be allowed 1 of that unit.
They can be any type of unit; Infantry, Daemon, Support, and in a special case even a Structure.