1. A wizard is someone who experiences sufficient skill in more than a single magical discipline to combine them in united and effective form. The results of wizardry are unobtainable through the exercise of a single one of such disciplines. Most Dragaeran wizards are of the House of the Athyra, or the House of the Dzur.
Zungaron gives a formal definition to Vladimir Taltos as "the art of uniting with and controlling disparate forces of nature to produce results unavailable from, or more difficult to obtain with, any single arcane discipline."
Some Teckla believe that wizards keep their souls in a box to keep them safe.
2. A more localized term which presumably originated from the first meaning is used in Jhereg slang to refer to anyone who is especially good at exercising a specific and desired skill. e.g. "a safecracking wizard"