The "space" in a spellcaster's mind dedicated to holding a spell of a particular spell level. A spellcaster has enough spell slots to accommodate an entire day's allotment of spells. Spellcasters who must prepare their spells in advance generally fill their spell slots during the preparation period, though a few slots can be left open for spells prepared later in the day. A spellcaster can always opt to fill a higher-level spell slot with a lower-level spell, if desired.