I see these words often used interchangeably when they are very much not. When a material is "filled" be it an epoxy, polymer, concrete, paint, etc with some sort of fiber or thixotropic material that means that the filler is evenly dispersed thoughout the material and is often too small to see individual particles. E.g. molybdenum particles in chain lube. This is a moly-filled lube. You cannot see it, but it's there. "Reinforced" is when you CAN see the filler material. That's the classic carbon fiber look where you can see the weave of the carbon fabric in the epoxy. Concrete is also a great example of a reinforced material. Those pre-stressed steel rods or even carbon/fiberglass rods sometimes are the solid reinforcement. can a material be both filled and reinforced? Sure why not. Concrete can sometimes be polymer or rubber "modified". This is essentially the addition of rubber or polymer "fill" powder in a reinforced concrete structure. This can be done to change impact reactions, stiffness, wear characteristics, etc. /rant