  Entropy production is the crucial quantity characterizing irreversible
phenomena and the second law of thermodynamics. Yet, a ubiquitous definition
eludes consensus. Given that entropy production arises from incomplete access
to information, in this Letter we use Jaynes' maximum entropy principle to
establish a framework that brings together prominent and apparently conflicting
definitions. More generally our definition of entropy production addresses any
tomographically incomplete quantum measurement and/or the action of a quantum
channel on a system.
