Oh, but there IS a good definition of pornography:
It's a contraction of the greek words "porne" for prostitute and "graphus" for description/depiction.
From this follows the definition: The depiction/description of a prostitute's life.
Ergo: if it's single nudes, it can't be porn, however lewd the picture itself may be(there are other names for that); There need to be at least two parties in a prostitution-relationship. No maximum required, btw.
If I really want to be anal about it, I could state that an exchange of money,goods or services between the participating parties is also a requirement.