Links joined so that each holds the next. A chain is exactly as strong as its weakest link and no stronger.
Old French chaeine, from Latin catena (a chain, a fetter), of uncertain further origin.
Strength is a property of the worst part rather than the average of the parts, so a chain built from ninety-nine sound links and one poor one is a poor chain — which is why auditing one means hunting the weakest point, not admiring the links that already hold.