The length of time an item remains in your inventory if not destroyed by you or sold (if possible). After this time, the item destroys itself. Different from the standard buff, debuff, or spell duration.
A duration may be given as a span of real time, in which case the item disappears after that amount of time has passed in the real world (as opposed to in the game world). When items with a duration are stackable, units added to an existing stack will obtain the duration of that stack. Thus the timespan of items can be extended so long as items of a newer duration can be obtained.
A duration may also be expressed as "requires (event name)". Items of this nature will disappear when the event concludes. As all instances of that item are removed at the same time, their duration cannot be extended by acquiring new items of that type.
Items with durations may not be mailed nor placed on the auction houses.
Various items received during holiday event have durations.
Examples of items with durations:
Limiting items with a lifespan of realtime, or sometimes played-time, is well established. Limiting items by "requires (event)" is relatively recent (2009 events and later). It has fairly often been the case that items for new events have been given a Duration without the item tooltip being appropriately updated before the event is released. These items are often updated during later Hotfixes or Patches.