Sexuality is determined by four things: the person's chromosomes, internal sex organs, external sex organs, and psyche. Bisexuality covers all of the area here seeing as sexuality is on a continuum and not simply classified as a binary stagnant thing. Some may argue hermaphrodites, pseudo-hermaphrodites, and other IS people don't fit into one gender group. However, they consist of male and female sexual organs and also combinations of X and Y chromosomes. Regarding people that are transgendered, they are biologically of one sex regardless of how they manipulate their body through procedures or hormones.
Furthermore, it should be noted the difference between omnisexuality and pansexuality. Omnisexuality's definition includes people that are bisexual and have pedophilia, necrophilia, zoophilia, etc. Further clarification of these words can be found in the easy to read "Sexologies" journal from the European Federation of Sexology.
The "bi" particle in "bisexual" simply indicates that a person is attracted to both sexes. While there are arguably 3 genders (males, females, and those who associate with neither), there are only two sexes. Bisexual people generally are attracted to both sexes, which includes all genders, which includes everybody.
Bill: Hey Jone, the dogs are acting weird again.
Jone: yeah those Pansexual Cowboys just rode into town.
It is usually used by post-modern liberal queermos that think current "labels" don't fully encompass their complex brand of slutty--- I mean sexuality. So they make up their own words to sound interesting and cosmopolitan.
Person 2: No you ignorant person. That person is pansexual a fluffy nonsense word that I just made up. Pansexuals are so full and dripping with love that they can't help but get it on everyone. Better get a hepatitis test.
"No, that ignant, I don't see people at all, I'm pan sexual"
"So you love ghosts?"
"Yes, we make love with Ouija boards"