Definition: Pokemon that has high attack (Physical, Special, or Both) and can learn high powered moves that counters Tanks and Walls. Consequently Bunker Busters have low defense and/or health. Tow examples are Eboar and Porygon-Z
For Eboar: More of a physical Bunker Buster due to higher ratio of physical attacks that special attacks and Emboar higher higher attack than special attack.
For Porygon-Z: More of a special Bunker Buster due to it's high special attack and can learn various move types to counter most, if not all, combinations of types.
Any questions, comment. -AND- Here my question that I have asked that got all the roles and a 'Bunker Buster' role was not listed in the answer. http://pokemondb.net/pokebase/122105/is-there-a-link-where-it-shows-the-acronyms
These are called wall breakers. They are already a role so sorry about that. They are Pokemon similar to Porygon and Emboar who just smash everything very hard and can "break" walls. Other Examples are Victini with V-create