What is an ABD? Can they use the prefix "Dr".?

ABD stands for All But Dissertation. That means a PhD candidate who has
completed the coursework requirements for PhD but who has not completed
thesis component. Since PhD dissertation, it is called PhD thesis counts 50%,
an ABD is only 50% PhD. An ABD is still in the pipeline and she cannot call
herself "Dr. X", she is still "Ms. X". So the ABD status is not equal to a PhD
as she may still end up without a PhD. If an ABD claims he or she has earned
a PhD then that is not an innocent mistake but a serious breach, that will tell
the integrity of a man. Some graduate schools may offer Master of Philosophy
to a PhD candidate whose work which is reported in the PhD thesis is not up
to the work of a PhD. But ABD is not a PhD.

See the Wikipedia entry below for the definition of ABD status:
http://en.wikipedia.org/wiki/Doctor_of_Philosophy