It is the constant presence of a disease within a region, making its spread more predictable.
Difference between endemic, epidemic and pandemic (World Economic Forum)
(AlJazeera)
How Pandemics Spread
How Pandemics Spread by TED Ed 2012 [7:59] (note the date, it was released prior to COVID-19) Description: Dig into the history of pandemics to learn how viruses and disease spreads and what we can do to stop future outbreaks. In our increasingly globalized world, a single infected person can board a plane and spread a virus across continents. Mark Honigsbaum describes the history of pandemics and how that knowledge can help halt future outbreaks.