Because of the now world-wide paranoia about anything that may excite paedophiles, anything to do with nudity of under 18's seems to arouse alarm in the media. Now Australia is going back down that route, having for a time been probably the most broadminded of English speaking countries. The alarm and bleating from the prudes about some very tame, unexplicit photos by photographer Bill Henson of a 13 year old girl topless (she didn't even have any tits that you would notice), started quite a debate in the media. The Prime Minister even weighed in before any charges were laid. Fortunately no charges were laid because the case should have been seen to be prejudiced due to all the undue attention and comments. If the powers to be, and especially the religious zealots, would spend more time making a genuine attempt to end wars and violence the world would be better off; we will probably solve the problem ultimately by ruining the environment and destroying life anyway, so maybe it is too late.
I've heard any nude footage in Japan is considered pornography apparently. So, they must adhere to the law and feature only 18 and up models. It seems nowadays simple 'nudity' is labeled as pornography, if the nude one is below 18, it's illegal. For lack of a good description, I guess an example of something erotic could be anything from simply posing in suggestive ways, you know. Or sucking on something. Or 'sexually' and sensually eating something slowly, slowly and sensually, let's say like perhaps a banana.
Yes, they are illegal. They were not before 1999 when the current CP laws were enforced. No, they are not. CP is defined by the Japanese laws roughly as images depicting sexual intercourse with minors and full or partial nudity which stimulates sexual desires of the viewers. So erotic is defined as a quality of something that excites and stimulates sexual desires. The definition of CP by the Japanese laws is ambiguous, thanks to which some questionable Junior Idol material can exist.
Things are pretty strange worldwide, nowadays. You can buy a David Hamilton book, even Pretty baby no prob, but you can't have pictures in your HD and so on... Weird.
Some of the works by Garo Aida, Shinkosha, Kingdom, God etc in which "partially clothed" girls wearing T-backs and skimpy bikinis and are posing in suggestive and lascivious manners and doing actions viewers could associate with sexual conducts . If the definition of CP is literally interpreted, these works could be considered CP.
Recently there was a case in Australia were someone was convicted of child pornography for have images of the Simpsons engaged in sexial acts (lolicon). The judge ruled that explicit drawings of children (bart and lisa) was illegal. So anyone who likes lolicon. It is illegal in Australia.
Although not always not the the most up to date source wiki is a fountain of information: http://en.wikipedia.org/wiki/List_of_pornography_laws_by_region