Is 'active' spark gap in air always plasma? When air becomes ionized enough to be conductive like in spark gap, is it always plasma or am I misunderstood something? Eg does Tesla Coil produce plasma?
 A: It is indeed a short-lived plasma as evidenced by the fact that it becomes electrically conductive. 
Most of the chemistry and dynamics of our familiar world turns out to be the physics of electrons and kinematics of nuclei: we have for example covalent bonds and ionic bonds defined purely in terms of how electrons are sticking to this nucleus or that nucleus and holding the nuclei together. Now in particular we have metallic bonding, which occurs when some electrons get "shared" among a bunch of nuclei, usually in a dense crystal lattice. Plasmas, those states of matter seen in lightning bolts and the Sun, are defined as those states where electrons become "shared" among a bunch of nuclei which are moving around in a gaseous state: the basic result is that such objects become conductive the same way metals do. Other phenomena, like light emission for example, are much more about the particulars of the situation: in this case the spark releases a lot of extra energy as it tears these electrons from their nuclei, and that extra energy is very quickly reradiated as light as the electrons "fall back" to their original place.
In fact in astrophysics you get these study of "plasmas" (free-floating ion clouds) which have a lot less to do with the high temperatures of the Sun or the dielectric breakdown of the lightning bolt, and simply mean that there are a lot of freely-moving particles, but some of them are charged. It really is the freedom of charge transport that makes something a "plasma."
