The 13 Kohanic Cities are the 13 cities/villages and their respective peripheral territory listed in the Book of Joshua (Joshua 21:13-21) as having been allocated by Elazar and Joshua to the kohanim (Israelite priesthood) and their families. The Kohanic cities are a sub-set of the 48 Levitical cities allocated to sections of the Tribe of Levi.
References
13 Kohanic cities Wikipedia(Text) CC BY-SA