ISM in Street Fighter Alpha/Zero 3 refer to the three varieties of super gauges that the player can pick, that greatly effects the properties of their super combos. ISM itself is explained by the official guide to stem from the English suffix '-ism' "that signifies a ideology (主義)", hence it's "no matter the character, it's the combat style (ideology) that you prefer".
X-ISM - Derived from 'Super Street Fighter 2 X', and is subtitled as シンプル (simple) in that it gives you one powerful super combo, harkening back to the implementation of super combos in SSF2.
Z-ISM - Derived from the Zero moniker of the Alpha series in Japan. Three levels of super combos. Referred to as スタンダード (standard). This is changed in the western releases to "A-ISM", i.e. "Alpha-ism".
V-ISM - Derived from "variation", and it is also subtitled as such in the guide as バリエーション (variation), essentially custom combos, where the guage's level determines how long your combo's duration is.