  The middle corona, the region roughly spanning heliocentric altitudes from
$1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical
transitions and processes that govern the behavior of coronal outflow into the
heliosphere. Eruptions that could disrupt the near-Earth environment propagate
through it. Importantly, it modulates inflow from above that can drive dynamic
changes at lower heights in the inner corona. Consequently, this region is
essential for comprehensively connecting the corona to the heliosphere and for
developing corresponding global models. Nonetheless, because it is challenging
to observe, the middle corona has been poorly studied by major solar remote
sensing missions and instruments, extending back to the Solar and Heliospheric
Observatory (SoHO) era. Thanks to recent advances in instrumentation,
observational processing techniques, and a realization of the importance of the
region, interest in the middle corona has increased. Although the region cannot
be intrinsically separated from other regions of the solar atmosphere, there
has emerged a need to define the region in terms of its location and extension
in the solar atmosphere, its composition, the physical transitions it covers,
and the underlying physics believed to be encapsulated by the region. This
paper aims to define the middle corona and give an overview of the processes
that occur there.
