A problem with your spell check.
Seriously, "womyn" was coined by feminists (I believe maybe Mary Daly but don't quote me) as a way to point out the sexism inherent in the very language we speak, and to reclaim that language for ourselves. It is an abstract attempt at levelling the playing field, that while I am sympathetic to it, I understand that it would only appeal to linguists.
Now, as to the more philosophical aspect of your question: What is a womyn? In simplistic terms, a womyn (or, woman, if you prefer) is an adult female homo sapien sapien, possessing a uterus, ovaries and a vagina. However, is that truly what a woman is? If that were the case, how come we have all these advice writers telling us how to act more womanly? And, as some of the previous answerers have so clearly demonstrated, it is rather easy to lose one's womanly status and become a "lesbian" or a "feminazi." So, there must be something more to being a woman than just possessing the requisite sex characterstics. Well, in patriarchal terms, a woman is a negative man. That is, men are active, logical, strong and level-headed. Women are everything that a man is not, therefore: passive, irrational, weak and emotional. But this does not really hold true either, because there are some that are born with female genitalia that are logical, actie and strong; and some that are born with male genitalia that are passive, weak and emotional. Does that mean that the former are men and the latter are women? That does not make sense. So what the heck is a "woman" anyway? I can only conclude that there is no such thing as a "woman."
So, your answer is, there is no such thing as a "woman" or a "womyn." The whole thing is just a product of your imagination.