The D&D Wizard has the power of Gandalf or (Malory's) Merlin, the versatility of Dumbledore or (Disney's) Merlin, and the frequency of Doctor Fate or (DC Comics) Merlin. That is, phenomenal cosmic power (up to and including literally rewriting reality), the ability to perform almost any act one might want to if you just have enough time to research the appropriate spell, and a power supply that is theoretically limited but practically inexhaustible because it's always at least somewhat under the Wizard's control.