Geez, even I can see a problem here already!
Hifi - short for 'high fidelity'. An audio system capable of reproducing sound, especially music, with as much fidelity (accuracy) as possible.
Audiophile - someone who is interested in and loves high fidelity sound reproduction. (from Dictionary.com)
Stereo: any device capable of producing or recording 2 channel sound. It can even be a kid screaing in each of your ears...
The actual definition of stereo is far more complicated, and can even refer to THREE dimensions, such as the stereochemistry of molecules, and stereoscopic viewers, for seeing photographs in 3D