到底什么城市是十八线城市？

经常在这里看到有人说自己在五线、六线甚至十八线城市，我很好奇这一般是指什么城市。 有没有人举下具体的例子，什么是十八线城市？有什么特点让人称其为“城市”而非别的什么。

Wiki 上的 City 定义是：

A city is a human settlement of notable size. It can be defined as a permanent and densely settled place with administratively defined boundaries whose members work primarily on non-agricultural tasks. Cities generally have extensive systems for housing, transportation, sanitation, utilities, land use, production of goods, and communication.

看起来人口密度高、主要产业非农业是重要特征。