Proposal: *Arduino *
There's Arduino software, which is a simplified software development environment. If you try to do anything efficiently or complex you quickly discover the limitations this environment places on you, but it's fine as an introduction to physical computing. Is this what Arduino means, and does it exclude the hardware?
Then there's Arduino board specifications, which basically come down to the header layout (the latest being the R3 layout), but then there are Arduino boards that don't conform, like the Nano. These boards don't require the Arduino environment to program them; you can, for example, use the AVR software suite. Is this what Arduino means, and does it exclude the Arduino toolset?
Of course, having developed some Arduino code, you can then build your own AVR hardware on perfboard or a custom PCB. Is this in scope?
Does the Processing language get a look in, given it's often coupled with Arduino examples?