Stereo Headphones

What makes headphones stereo or not? This seems like a simple question with a simple answer but it is far from easy. So lets start with the definition. Stereo: audio signals are routed through 2 or more channels to simulate depth/direction perception, like in the real world. So when it comes to the actual sound, it does matter how the file is created and the quality of the headphones isn't really determined by the two channels. The idea is replicating "the real world".