Inter-symbol interference (ISI)

Inter-symbol interference is the ghost of the previous bit haunting the current one. Because a lossy channel smears each sharp pulse into a long, drawn-out tail, by the time bit number 10 arrives, the fading echoes of bits 7, 8 and 9 are still hanging around and adding to (or subtracting from) it. It's like talking in a room with a long reverb: each word blurs into the next until you can't tell where one ends and the next begins.

ISI is the number-one enemy at high speed: it makes a bit's voltage depend not just on whether it's a 1 or 0 but on what the bits around it were, which closes the eye diagram and pushes up the bit-error rate. The whole equalization toolbox is an anti-ISI campaign — a feed-forward equalizer cancels echoes that arrive before the main bit (pre-cursor), and a decision-feedback equalizer subtracts the echoes that trail after it (post-cursor).