I noticed you have a DSA8200 scope and the TDR modules 80E10+ 80E08 and 80E04. What is TDR?

I noticed you have a DSA8200 scope and the TDR modules 80E10+ 80E08 and 80E04. What is TDR?

Time Domain Reflectometry - a measure of reflection in an unknown device, relative to the reflection in a standard impedance.
A known stimulus is applied to a standard impedance and is propagated toward the unknown device, reflections from the unknown device are returned toward the source and measured.