We have taken definitions from this excellent article and reproduced some below https://www.yalemedicine.org/stories/covid-19-glossary/
Spread of disease
When a disease—and the virus that causes it—begins to spread, epidemiologists (who are considered the basic scientists of public health) take notice, looking for the frequency, patterns, and causes associated with it. Below are definitions of a few of those epidemiological terms that you may hear or see reported in the news, especially as they relate to COVID-19.
The baseline, or expected, level of the disease in the community—meaning it always exists, like the common cold and flu, which are usually at low, predictable rates.
This refers to a sudden increase in the number of cases of a disease, above what is typically expected in a particular area. COVID-19 is thought to have reached epidemic proportions in China in mid-January. “There is not really a date because there is no background [endemic] activity of this novel coronavirus in humans,” says Dr. Meyer.
This shares the same definition as epidemic, with one exception—an outbreak usually refers to a more limited geographic area. COVID-19 started as an outbreak in Wuhan, the capital city of the Hubei province in China at the end of December 2019, when the Chinese government confirmed that it was treating dozens of cases of pneumonia of unknown cause.
An epidemic that has spread over several countries or continents, impacting many people. Pandemics typically happen when a new virus spreads easily among people who—because the virus is new to them—have little or no pre-existing immunity to it. COVID-19, which was declared a pandemic by the WHO in early March, is the first pandemic known to be caused by the emergence of a new corona-virus.