What is the coastline paradox?

The measure of the length of a coastline depends on the level of detail at which the measuring is done.

For example: if you measure the coastline of a roughly circular island, at a low level of detail, where little ins and outs along the coastline are ignored, the length will be shorter than if you measure every in and out at a fine level of detail, because each of these little ins and outs adds to the length. The closer or finer you measure, the longer the measurement. This is called the "coastline paradox".