Used rear wheelarch liner / splash guard, right hand rear (drivers side or offside on UK cars), Mazda MX-5 & Roadster mk2 1998-2005.
This is the part that goes in the rearward part of the rear wheelarch (on the end of the bumper), behind the rear wheel.
Mazda call this a splash shield.
UM00707. NC1050341 (also N06650341A).