Why are Arduino peripherals called shields?
I doesn't look like they've registered a trademark for it ("shield").

The mostly obvious explanation is the mechanical position of the daughterboard. One book says:


  These boards are called “shields,” because they usually fit over the top of Arduino like a protecting shield.


The setup is by no means new. I had an ancient (1990-era) video card that had a 2MB addon-VRAM card (for a total of 4MB); it looked like the image below