I don't really think there is a universal term. There are plenty of adventures where a certain NPC fits the role of the area boss especially in site-based adventures (adventures that take place almost wholly in a specific location such as a dungeon, ruin, village, etc). And even I informally refer to them as a boss sometimes.

But in an actual module (at least all the ones I have written or read/run) they tend to have extensive write-ups on their motivations and it's pretty clear they are the leader or most powerful NPC enemy present so you never really need them to be explicitly labeled as the "boss".