Abstract <p>The generation of a highly-ionized cathode plasma in millimeter air gaps at pressures of about 100 Torr is investigated. It is shown that this plasma appears explosively at once in the moment of the electric breakdown of a discharge gap, with its characteristic velocity of expansion into a hemispherical region relative to the approximate explosion center on the cathode surface being of ~10⁵ to 10⁶ cm/s. The plasma initiation occurs almost always on the lateral surface of the pointed cathode slightly below its tip. It is established that the observed plasma has a high electron density of around (1–6)×10¹⁹ cm⁻³ almost tenfold exceeding the plasma electron concentration produced by complete dissociation and single ionization of air particles at a pressure of ~100 Torr. The parameters of the explosion of the cathode substance, the composition of the cathode plasma, and the fundamental and applied aspects of the observed phenomenon are discussed.</p>