  This paper addresses the following main question: Do we have a theoretical
understanding of entanglement applicable to a full variety of physical
settings? It is clear that not only the assumption of distinguishability, but
also the few-subsystem scenario, are too narrow to embrace all possible
physical settings. In particular, the need to go beyond the traditional
subsystem-based framework becomes manifest when one tries to apply the
conventional concept of entanglement to the physics of matter, since the
constituents of a quantum many-body system are indistinguishable particles. We
shall discuss here a notion of generalized entanglement, which can be applied
to any operator language (fermions, bosons, spins, etc.) used to describe a
physical system and which includes the conventional entanglement settings
introduced to date in a unified fashion. This is realized by noticing that
entanglement is an observer-dependent concept, whose properties are determined
by the expectations of a distinguished set of observables without reference to
a preferred subsystem decomposition, i.e., it depends on the physically
relevant point of view. This viewpoint depends in turn upon the relationship
between different sets of observables that determine our ability to control the
system of interest. Indeed, the extent to which entanglement is present depends
on the observables used to measure a system and describe its states. This
represents a most conspicuous advantage as will be highlighted by the
condensed-matter application we will discuss.
