There is no "correct" definition for the first two; they are not used consistently even in 'authoritative' publications. A *reasonable* definition would be:

Shock: to add (to a pool or spa) either more than the normal amount of halogen (chlorine, bromine, iodine) or peroxide (PHMB pools!) oxidizer OR to add a supplemental non-chlorine (potassium monopersulfate, etc.) oxidizer in order to chemical oxidize contaminants.

Super-chlorinate: to 'shock' using a halogen. (See above).

Breakpoint chlorination: to chlorinate to a calculated ppm, based on the measured presence of DPD #2 reactants, in an effort to oxidize chloramines.

This term -- breakpoint chlorination -- seems to be based on Thomas Palin's theories about chlorine oxidation of chloramines found in treated water. This theory seems to describe, with reasonable accuracy, what happens when the chloramines are the result of simple ammonia compounds. It does not describe at all, what happens when the chloramines present are the more complex result of the less simple nitrogen sources, such as urea, creatnine, etc. Since these more complex contaminants are the typical ones present in a pool, "breakpoint chlorination" does NOT occur as described in most 'official' pool literature.

Hope this helps.