Insertion loss
Beginner
How much weaker a signal gets by the time it reaches the far end of a wire.
Novice
The fraction of signal power lost along a channel, given in decibels (dB). 10 dB means one tenth of the power arrives; 20 dB means one hundredth. Copper loss grows with both length and frequency.
Expert
|S21| in dB, quoted at the Nyquist frequency. For twinax and PCB traces it is roughly (skin effect) + (dielectric loss) per unit length, so it scales linearly with length and faster than linearly with lane rate. Standards specify end-to-end budgets, such as 28 dB at 26.56 GHz for 100 Gb/s lanes.