Hi,
It simply means that there is no path for DC current, i.e. a
metallic connection.
not really: it means that there is NO DIRECT PATH for any current
at all!
Direct path means any kind of conductive (or semiconductive)
material. By this definition, the magnetic coupling of a
transformer provides galvanic insulation.
With regards to saftey standards (like ISO 60065 or UL1650 ...) the
term galvanic insulation is mostly coupled to a clearly specified
value of withstand voltage, that the insulation must keep without
any wrong behaviour:
For a mains transformer (PRI to SEC) this is:
1500Vacrms or 2120Vdcpeak for basic insulation
3000Vacrms or 4240Fdcpeak for reinforced insulation
By this you can see, that also AC currents are not allowed to run
directly from one side to the other. These tests are made with one
probe connected to the (shorted) primary windings and the other
probe connected to the (shorted) secondary windings.
Regards
Michael