A Common Boss is any Leader/Boss enemy NPC that has no specific importance to the storyline, nor to any requirements regarding an area.

In other words, these Bosses have no bearing on the game world beyond the fact that they exist at that point in time. This is contrasted by Major Bosses (who are key figures that move the storyline ahead), and by Sub-Bosses (who are more like Common Bosses, but whose defeat is required in order to trigger some event or to fulfill some sort of quest).