Per day

This means that a class feature or item function etc can be used up to a maximum number of times each day. This is usually taken to mean between midnight of one day and the midnight of the next, though it could be used to mean a maximum usage within a total 24hrs period, or that each usage becomes usable again once 24hrs has past for each individual usage.

e.g. or
 * Barbarian rage 5/day means that barbarian rage can be used 5 times each day.
 * If a character has druid wild shape 2/day and uses both wild shapes he must wait until 24 hours since the first usage has past before he can wild shape again. If 24hrs has past since the first wild shape use but not 24hrs since the 2nd attempt the druid would only have regained a single usage.

NWN comparison

 * Characters in NWN regain all per day uses when they rest.