Plain English Meaning
A series of small bad choices that link together. On their own each one seems harmless, but stacked up they lead toward an accident. Spotting one link early and fixing it usually stops the whole chain.
Full Definition
A sequence of small, individually minor errors or poor decisions that build on one another during a flight, each one narrowing the pilot's remaining options and increasing the likelihood of an accident. Recognizing and breaking the chain at any link can prevent the final outcome.
Context Anchor
Used in pilot decision-making, safety training, accident discussions, and post-flight review when looking at how several small choices combined into a larger problem.
Derivation
The 'chain' image comes from the idea that no single link causes the failure -- it is the connected series. Aviation safety literature borrowed this from accident-investigation work showing that crashes almost never have one cause; they have a sequence.