What is a "Fiend"?

[pres man]

And by extension, I guess I could ask, what is a "Celestial"?

How are the descriptions defined?

Is any outsider with an evil subtype considered a fiend?
Are only demons and devils, true fiends?
Outsiders with the evil subtype, but not necessarily native to one of the evil aligned planes (such as a howler), could they be fiends?

As well as similar questions for Celestial beings.

[wraithstrike]

Fiend is not an official game term, and is normally only attributed to outsiders with the evil subtype such as demons and devils..

[Generic Villain]

Fiend has always been a catch-all term, though it's pretty ambiguous. I've seen it used throughout different iterations of D&D, and it's usually a term for intelligent evil outsiders. I most often picture demons/tanar'ri, devils/baatezu, and daemons/yugoloths, but it could really be almost any native of one of the evil planes of existance.

Celestial is equally ambiguous, though I generally picture anthropomorhpic beings. Angels, agathions, guardinals, and the like.

In short, there's no set definition for either "fiend" or "celestial."

[Todd Stewart]

Are they evil, represent some ideal associated with that alignment, and live on an evil aligned plane?* That's my general feeling on the topic for what seperates a fiend from something like efreet which might be evil aligned, but are more a representation of an element than an alignment.

*caveat here are things like Rakshasa which I consider to be true fiends, but which on Golarion are a special case because they're bound to the Material plane. I always viewed them as Acheron natives in 2e and 3e, and thought that the (native) outsider status was originally a poor decision. The Golarion explanation of that status however is a very good, flavorful reasoning.