What term is used to describe a widespread outbreak of a disease across large regions?

The term that describes a widespread outbreak of a disease across large regions is "pandemic." This term is specifically used in public health to indicate a disease that has spread over multiple countries or continents, affecting a significant portion of the population. A pandemic suggests not just the presence of a disease but its extensive reach and impact, surpassing local or regional confines.

In contrast, terms like contagion, epidemic, and outbreak refer to different scopes or contexts of disease spreading. A contagion can refer to the spread of any infectious disease but does not imply the geographical extent. An epidemic is typically confined to a specific region, indicating an increase in disease cases beyond what is normally expected in that area. An outbreak, while it may refer to a rise in instances of disease, usually describes a situation in a limited area or specific population rather than the broad and widespread nature of a pandemic.