  An entangled state is bound entangled, if one cannot combine any number of
copies of the state to a maximally entangled state, by using only local
operations and classical communication. If one formalizes this notion of bound
entanglement, one arrives immediately at four different definitions. In
addition, at least three more definitions are commonly used in the literature,
in particular so in the very first paper on bound entanglement. Here we review
critical distillation protocols and we examine how different results from
quantum information theory interact in order to prove that all seven
definitions are eventually equivalent. Our self-contained analysis unifies and
extends previous results scattered in the literature and reveals details of the
structure of bound entanglement.
