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document updated 15 years ago, on Mar 24, 2008
Impedance is the AC analog of resistance. (though unlike in DC, capacitors and inductors modify a signal's phase, and thus also impact impedance)

Impedance discontinuities cause AC waves to be reflected back towards the source. That one fact is why so much engineering time is spent on:

In digital systems, the eye pattern is a great way to verify that appropriate margins are being held to.