Fiend
In D&D and Pathfinder, a fiend is a negative supernatural entity—what we would generally call a demon.
The term is a bit weird since, again, fiends are what we'd call "demons." The term demon, however, is reserved in D&D and PF for one specific type of fiend: a Chaotic Evil race hailing from the Abyss.
Therefore, Asmodeus by this definition would not be considered a demon. He'd be a fiend—specifically a devil (a Lawful Evil fiend). This is especially weird since Asmodeus IRL is referred to as a demon.
To make matters even more confusing, you may hear various NPCs (and even me) referring to all sorts of fiends as demons. Fun.
Etymology
The word comes from the Middle English word for "enemy" or "demon," which comes from Old English feond ("enemy").
Main types
The three main types across both D&D and PF are devils, demons, and yugoloths (aka "daemons"). These three groups align with the three Evil alignments: devils are Lawful Evil, demons are Chaotic Evil, and yugoloths are Neutral Evil.
Basically, in ascending order of "how badly do I NOT want to run into one of these guys," it goes: devils, yugoloths, demons.
