What do epidemiologists mean by the term 'population'?

What do epidemiologists mean by the term 'population'?

The population refers to entire group of people and it can have several definitions, depending on the frame of reference that one works with. For example we can speak of the population of the world; we can speak of the population of a specific country. Sometimes, we refer to the population of a state or a county. Or it can even be important sub-group, such as a University population of the students who are attending the University, or almost any group as defined by the epidemiologist.