In the Eberron setting material "fiends" play a central role, especially during the Age of Demons. References to "fiends" are almost always followed up by a reference to rakshasas, and obviously rakshasas play a more central role in Eberron than they do in other D&D settings . However, does the term "fiends" refer to all evil outsiders; devils, demons, and rakshasas, only rakshasas or some other grouping of evil beings?
Although the term is never defined specifically, the evidence provided tells us that, in Eberron, "fiends" is the generic term for demons, devils, and other evil outsiders that were spawned from Khyber at the dawn of the world (ECS, page 224). They seeped up from the cracks in the earth, from volcanoes and the deepest parts of the sea (p.158). They are capable of possession (p.100) and are led by outsiders more crafty than they are.
It can be inferred that, since the cosmology and planar structure of Eberron differs from the typical Great Wheel of D&D, the terms "demon" and "devil" aren't relevant to the setting. "Fiend" makes a nice generic.
"Fiend" in D&D 3.5 refers to any evil outsider.
The term is defined in the Monster Manual, under Types and Subtypes:
Evil outsiders are also called fiends.
The only change in Eberron is that night hags are neutral outsiders rather than evil (Eberron Campaign Setting p. 305), and thus are not classified as fiends in that setting.
While certain creatures gain or lose the Extraplanar subtype in Eberron to account for differences in the planar cosmology (Eberron Campaign Settting p. 100), this doesn't actually change their type to or from Outsider, and so doesn't affect which creatures are considered fiends.