The accepted definition of the "deep stall" or "super stall" is that of Davies', which refers solely to the blanked T-tail designs.
The term "deep stall" (and the phenomena) already existed at least 3 decades before the first T-Tail was invented.
Basically what it means is stable flight in the region between the first Clmax (around 10° AoA, depending on the airfoil and configuration) and the second Clmax, which naturally occurs around 45° AOA for every airfoil. There is always a second "post stall" region of positive Cl over AoA slope, and if the horizontal stabilizer allows to get there, the aircraft can be flown stable in that region with lift and drag of similar magnitude, hence with flight path angles somewhere between 30 and 45 degrees. Depending on the overall design this condition can be so stable, that elevator efficiency is not enough to get out of that. The T-Tail is the most often occurring example for such configurations, but not the only one.
BTW, deep stall with T-Tails is not stable with the Tail
in the wake of the wing, but with the Tail on the rear boundary of the wake. It is not so much because the elevator looses efficiency, it is because the flight condition is so extremely stable if any pitch up results in the tail leaving the wake and hence producing lift again and a nose down pitching moment, while pitching down results in the tail fully entering the wake and hence producing less lift and a nose up pitching moment. The Cm curve is so steep in that region, that shifting it up and down due to elevator deflection does not change the pitching moment significantly.
Canard configuration aircraft can even have stable deep stall around 230° AOA (inverted backwards...) as discovered during the SpeedCanard flight test...