The length of time a spell or effect lasts for. For spells, it is often based on caster level, but some spells have a fixed duration (e.g. the Summon Monster spells, whose duration is 10 minutes).
Since U14, numerous low-level spell buffs with a duration of 1/minute per caster level have a minimum of 5 minutes.
Use Extend Spell to increase the duration of buff spells.
Quests are categorized by duration, ranging from very short to very long; see Quests by duration. This information used to have to gameplay impact until the introduction of Reaper difficulty, where it affects reaper XP.