Definition: A line segment from the center of a regular polygon to the midpoint of a side.

Try this
Adjust the polygon below by dragging any orange dot, or alter the number of sides. Note the behavior of the apothem line shown in blue.

The apothem is also the radius of the
incircle of the polygon.
For a polygon of n sides, there are n possible apothems,
all the same length of course.
The word apothem can refer to the line itself, or the length of that line.
So you can correctly say 'draw the apothem' and 'the apothem is 4cm'.

Each formula below shows how to find the length of the apothem of a regular polygon.
Use the formula that uses the facts you are given to start.

Apothem given the length of a side.

By definition, all sides of a regular polygon are equal in length. If you know the length of one of the sides, the apothem length is given by the formula:

Since irregular polygons have no center, they have no apothem. In the figure above, uncheck the "regular" checkbox and note how there can be no center or apothem.