Actually, Safer Sephiroth is the One Winged Angel form. The form spoken of in DarkZero's writeup is the third form, but this form is perhaps more properly considered part of the ending than an actual boss, as it is impossible to lose the fight against him.
It is also possible for this form of Sephiroth to fight back, if you stand there and do nothing for long enough. However, this attack will never be strong enough to kill you, and will automatically provoke a counterattack from you (quite an anticlimax, in comparison to using Omnislash).
Interesting to note is that unlike the first two forms (Bizarro Sephiroth and Safer Sephiroth), this third form has no name. In reference to his state of dress, some people call him Shirtless Sephiroth, but this is unofficial.
As to Safer Sephiroth, however, the origins of the name are unknown. Some believe it is a corruption of "Seraph" via "Seraf" (FF7 was very keen on naming things using corruptions of words). There's also a theory that Safer is the name of an Egyptian deity, but I can't find any information to back this up.