Every electronic product with an input has an impedance, though I suspect few of us pay much attention to it.
Understanding input impedance is valuable for those connecting equipment or making purchasing decisions.
Input impedance is defined by Wikipedia as, "... the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is external to the electrical source."
What a lot of gobbledygook. Let me see if I can put it in more understandable terms.
Input impedance determines how easy or hard the source equipment works to supply music. The lower the number, the harder it works.
Over the next couple of days we'll look at some of the use cases and what they mean.