I am sure you will probably get an infinite amount of responses to that question. I, normally, classify tech diving as any type of overhead diving. By overhead, I mean:
1. Physical: You cannot make a direct ascent to the surface because of some tangible barrier.
2. Virtual (EDIT: visual is too vague): You have decompression obligations that make a direct ascent to the surface significantly more dangerous.
Additionally, more complex equipment, ie rebreathers, are in the realm of tech. You will hear most people discussing 'tech' as any form of diving that requires more extensive planning, training, and experience than recreational diving.
People that go beyond recreational limits without training, planning, and experience, are not doing tech dives nor are they tech divers. They are stupid.