There is no precise definition, but for instance:
A microphone rated at 150 ohms to 300 ohms is considered low impedance.
About 600 to 2 000 ohms is medium impedance,
and 10 000 ohms or more is...
That depends on the output impedance.
In electronic we use voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times...
To get all the voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than...