CRS
Coordinate Reference System
Object that defines coordinate reference systems for projecting geographical points into pixel (screen) coordinates and back (and to coordinates in other units for WMS services).
Leaflet defines the most usual CRSs by default. If you want to use a CRS not defined by default, take a look at the Proj4Leaflet plugin.
Note that the CRS instances do not inherit from Leaflet's Class object, and can't be instantiated. Also, new classes can't inherit from them, and methods can't be added to them with the include
function.