Apparently there is a lot of debate over the term. From the NY Times:
Generations of people have used the term to describe widespread epidemics of influenza, cholera and other diseases. But as the new H1N1 swine influenza virus spreads from continent to continent, it is clear that a useful definition is far more complicated and elusive than officials had thought.
And what is at stake is far more than an exercise in semantics. A clear understanding of the term is central to the World Health Organization’s six-level staging system for declaring a pandemic, which in turn informs countries when to set their control efforts in motion.