One of the two kinds of magic user in Dungeons and Dragons Third Edition. This group includes the wizard, sorcerer, and bard classes, among others. They gain their power either through long study and training, or through simple instinct.
Wizards are the arcanists who advance through study, maintaining large spellbooks of their accumulated knowledge. They tend to know a great many different spells, because they constantly seek out new ones to add to the book. However, each day a wizard must plan ahead for the spells he plans to cast, and cannot easily change his selections.
Other arcane casters cast spells by innate talent. A caster of this type does not require training or study, and need not plan his daily spells in advance. The drawback is that, lacking a spellbook in which to collect magical writings, he has a far smaller variety of spells available to him. (Bards and sorcerers fall into this category.)
Arcane spells with somatic components are difficult to cast while armored or encumbered. The more restrictive the gear, the more likely it will interfere with the required gestures. A wizard in leather armor will flub 10% of his spells, while one wearing banded mail and carrying a tower shield will screw up 85% of the time.
Arcanists in general tend to be more flashy and dramatic than their divine counterparts.