2. To burst violently as a result of internal / external pressure.
3. To cause to release energy or burst violently and noisily
4. to burst with great violence; blow up
Is that not how a space shuttle "Explodes"? Something has to trigger the shuttle to "Fall into a million pieces". If not Fire, then Pressure is the only other thing I can think of. Please correct me if I'm wrong. 
Ok, you asked for it.
A deflagration is a propagation of a combustion zone at a velocity less than the speed of sound in the unreacted medium. A detonation is a propagation of a combustion zone at a velocity greater than the speed of sound in the unreacted medium. An explosion is the bursting or rupture of an enclosure or container due to the development of internal pressure from a deflagration or detonation. As defined in NFPA 69.
So yes, you are wrong. Your number 2 is the physical and engineering explanation, 3 and 4 are colloquial use.
It is a major difference, if you could replay the end of the shuttle in slow motion. The ET does not explode - it first disintegrates (instead of bursting from internal overpressure) by loosing aft tank dome and intertank structure, and there is actually almost no chemical reaction at all. The white cloud you saw, was just the effect of aerodynamics and internal tank pressure.
If a perfect mixture of hydrogen and oxygen would explode inside the ET, you would instead see no such cloud at all - the tank would burst into shrapnel, the fireball would follow.
The fate of the shuttle was similar: There was nothing to explode inside it and also no stuff to detonate. Instead, loosing the external tank below it caused it to get ripped into pieces by pure dynamic air pressure, when being forcefully placed at unfavorable angles of attack. Without fractions of a second, you would see first the wings fly away and disintegrate, then the nose, pressure hull (with many parts of the payload bay) and finally the still running engines. All in about 0.2 seconds from the tank collapsing below you and the pressure hull being in free fall.
In slow motion, it would be like a giant tearing it into pieces.
An explosion would make the shuttle burst into parts from overpressure inside.
But that not even happened to the fuel tanks in the nose section: These just got destroyed, possibly be debris and the contents of the tanks deflagrated partially. From the coloration of the cloud there, most of the tank contents did not react.
