In radio communications, characteristic impedance (acoustic impedance or sound impedance) 
of a uniform transmission line is the impedance of a circuit that, when connected to the output terminals of a line of arbitrary length, causes the line to appear infinitely long. The SI unit of characteristic impedance is the ohm.
A uniform line terminated in its characteristic impedance will have no standing waves, no reflections from the end, and a constant ratio of voltage to current at a given frequency at every point on the line.
If the line is not uniform, the iterative impedance must be used.
The characteristic impedance of a linear, homogeneous, isotropic, dielectric propagation medium free of electric charge is given by the relation
where
- is the characteristic impedance
is the magnetic permeability of the medium
is the electric permittivity of the medium
is the speed of propagation in the medium
When the medium is free space, the magnetic permeability 
and electric permittivity 
of free space are used and this defines the universal physical constant, the characteristic impedance of free space :
- here
is the speed of propagation in vacuum.
See also
Source
Adapted from