What is Femininity (Do read this in a 'in my opinion' format)
What is a man, what is a woman when all biological aspects and influences are stripped? Body. Hormones. Their functions and effects annihilated. Then definitions take far more thorough thinking, and an answer may never be given. Nevertheless, even then someone will still say "But, I am a woman" -- through sense, a deep realisation that cannot be explained that easily by scientific connotations. Gender may be a possibility to define it.
But, let's return to the initial question -- after all, this was only a short introduction in order to build a 'stop' sign at the right path -- What is femininity, not what is female, no; what is feminine.
Let's assume that femininity is perceived as feminine. The butch lesbians serves as a nice example for this (let's pick a stereotype shall we): most likely not percevied as very feminine, but most likely also not seen as less of a woman than the lipstick lesbian (the lesbian part is a bit of a given, innit?). Nevertheless, the point is, as this example suggests, that there are standards constituted in order to determin what is feminine (gender behaviour) and what is not.
Since we already dropped hormones and all the other biological jazz that may affect gender behaviour, the previous constituted standards in relation to perception seem rather linked to stereotypical venues. In this case, gender behaviour or the focus femininity would be associated with prehistoric connotations as well as etiquettes handled over previous centuries. The common images of gentleman and lady, introductions of pink and blue show that certain standards were handled. Although fluctuation between (these set) masculine and feminine behaviours has become acceptable (and in some circumstances even applauded as well as promoted) there is still a common perception to which certain demeanour is ascribed.
Therefore, I'll conclude that all these personal opinions lead to one grande personal opinion, yes my opinion, that was already mentioned once:
Femininity is what is perceived as feminine by stereotypical standards.... (In my opinion, of course...)
The Mad Hatter strikes again! [insert goofy theme-song]........... Now I want more coffee.