A sleeping patent refers to a patent which is not seemingly exploited commercially. However, it can often be a strategic move by a dominant market player to deter entry by competitors.For example, Xerox has been alleged to have taken out patents over inferior technologies in order to block entry and protect its dominant position.
Food for thought: Sleeping patents can prevent democratization of innovation, hurting the human society as a result. A way around the sleeping patent problem is by developing laws which force the patent-holder to license the technology to others, if the patent-holding firm refuses to productize and commercialize the patent!
Also see Citation Analysis.