Crossing traditional gender lines. Almost all
societies today have a
dichotic gender paradigm, and through
stereotypes and other
methods work to make sure members of the
society stay within carefully
defined boundaries. One who does things
outside of these boundaries is
transgendered.
It is a blanket term, and many subgroups could be defined inside of it. Crossdressers, transvestites, and transsexuals are all considered transgendered. There are also people who do not want to be placed inside those groups, and only want to be referred to as transgendered. They often do some sort of gender blending, taking on attributes of both genders in striking ways.
Often considered wrong and/or immoral by people, usually due to religious reasons, but not necessarily - much of it may be due to challenging norms that are usually considered inflexible.