Shapefile Projection Format File
Shapefiles are used to describe points, polygons and polylines when used with geographical information systems and are geospatial vector graphics.
When a shapefile is saved its stores a number of files that make it up, one of these files uses the .prj file extension, these files are projection format files which contain coordinate and projection information, these files are text files.
How to open .prj files?
Software to open .prj files
What are .prj files