Is it a flu epidemic or a flu pandemic?

Answer 1

Flu can be both. A flu epidemic is when the incident rate (number of people being infected) substantially what is expected based on previous experience. A flu pandemic is an epidemic that has spread through human populations across a large area.

Swine Flu is a world wide disease and has been officially called a pandemic.

Answer 2

It is a cyclic seasonal trend during which infections by influenza viruses can be seen spreading into outbreaks. With more and larger outbreaks, the virus is spread wider and wider and can become what is called an epidemic, or even continue to grow into a pandemic.

An example is the 2009 pandemic of "swine flu" H1N1/09:

1. Cyclic Trend - Flu Season ~ A brand new virus had mutated into existence from a known strain of influenza in birds and, through a complex process of multi-species infections and viral mutations (see related questions), it began infecting humans. The influenza viruses usually spread in a particular climate, and the time of year during which that climate occurs in your area is the time of the flu season there (see related questions). The cyclic trend of the climate and corresponding flu season then came to your location and the new kind of virus began to be active and doctors began seeing patients for symptoms and signs of flu-like illnesses.
2. Outbreaks ~ Isolated cases of H1N1/09 viral infections began to be seen spreading in larger and larger areas in clusters. Each region or large area with spreading cases and increasing numbers would be called an outbreak. These began being identified and reported as outbreaks in regions in Mexico and the US in spring 2009.
3. Epidemic ~ The outbreaks of "swine flu" grew bigger and bigger and joined with other outbreaks to be called epidemics once they covered an entire state or country or other large geographic location.
4. Pandemic ~ The swine flu H1N1/09 epidemics continued to rapidly spread around the world and the World Health Organization declared a true pandemic situation in effect and the world united briefly to work on containing the spread. The virus is still circulating in isolated cases and outbreaks as of 2013 although the incidence is significantly lowered.

See related questions for more information.