A non-dissipative medium is a medium within
which a quasistatic process is reversible. That is:
inside it a process conducted infinitely slowly dissipates no
work, creates no entropy.
For example:
Moving an object from A to B within an ideal gas - a non-
dissipative medium. As the object moves it bats the gas
particles in front of it forward. These bang into other
particles; setting all into violent motion. (The temperature
rises immediately in front of the moving object.) The
same process, acting in reverse, slows down the particles behind
the body. (The temperature cools immediately behind the
object.) Thus the body gets struck more violently and more
often on its front face than upon its back. This would bring it
to a halt were it not supplied with work - pushed - to enable it
to continue its journey. The more slowly it is pushed from A to
B the less work is needed; dissipated into increasing the speed
of the average particle in the sea of ideal gas particles. In
the limit, when the body is moved infinitely slowly,
quasistatically, no work is dissipated - no entropy created.
For example:
Sliding an object, from A to B, along a wooden plank - not a
non-dissipative medium. Here the force resisting the motion
does not necessarily get less if the body is moved more slowly.
It does not tail away to zero in the limit of infinite slow
motion. Indeed, as it happens, static friction is greater
than dynamic friction. It would always dissipate some work to
get from A to B.