A pandemic is when a dangerous disease breaks out across the world - as opposed to an epidemic, which is localised. This used to be impossible, but due to the increase in travel technology, an outbreak in one part of the world can now quickly spread to a major city and from there to the rest of the world. The term has a connotation of referring to an especially bad outbreak, but it refers to the distribution of the event, not the severity.
Notable pandemics include the 1918 influenza pandemic, which was encouraged by large numbers of military personnel suffering from poor sanitation and traveling between continents, and the ongoing AIDS pandemic. A recent (if less fatal) disease which was called a potential pandemic in the media was SARS, which started in China and made its way to Toronto, Canada. The bird flu virus (H5N1) has been seen as a possible pandemic virus. Swine flu (H1N1) was classified as a pandemic during the summer of May 2009 by the World Health Organization - killing over 14,000 people. Some portions of the press seemed to think that the pandemic never actually happened, and that it was all scaremongering.