http://www.d20srd.org/srd/magicOverview/arcaneSpells.htm#preparingWizardSpells
Spell Slots

The character class tables show how many spells of each level a character can cast per day.

These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level.


http://www.d20srd.org/srd/magicOverview/divineSpells.htm#preparingDivineSpells
Spell Slots

The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level.