oomis1105:
Actually, an Ervah is someone who the Torah discribes with the term “Ervas ….. ” (that we lain on Yom Kippur Mincha). In the Gemorah, they are refered to collectivly as “The Arayos”, and in singular it would be “an Ervah” (IIRC).
Niddah is somewhat of a middle ground, not quite the status of a “real” Ervah (such as a sister in law, for example), but also not Muttar. I would have to do more research on the Gemorah there.