A design pattern to force a class to have no more than one instance at any given time
the playing card that is the only card in a suit held in a bridge hand as initially dealt a set containing a single member a single object (as distinguished from a pair)
A class that may not be instantiated more than once at a time, i.e. that implements the singleton design pattern
The singleton property on an entity allows that only one instance of the entity is ever instantiated trigger Triggers invoke operations upon the occurrence of some event Triggers can apply to entities, attributes, and relationships