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What is undefined behavior (UB) in C++? Why is reading an uninitialized variable undefined behavior? What happens if you dereference a null pointer? What are common causes of undefined behavior? Why is signed integer overflow undefined but unsigned overflow defined? How do you detect and prevent undefined behavior? What is the strict aliasing rule and how can violating it cause UB? How can undefined behavior cause 'impossible' optimizations and bugs? What is a data race and why is it undefined behavior?
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What is the difference between undefined, unspecified, and implementation-defined behavior?

Beginner

Answer

Undefined behavior has no constraints—anything may happen (must be avoided). Unspecified behavior has several allowed outcomes and the implementation need not document which (e.g., order of evaluation of function arguments). Implementation-defined behavior is unspecified but the implementation must document its choice (e.g., sizeof(int)). Only UB is dangerous; the others are just non-portable in defined ways.
Real-world example sizeof(long) is implementation-defined, argument evaluation order is unspecified, and a[5] on a size-3 array is UB.

Common follow-ups: Which must the implementation document? | Which one is dangerous?

Compilation & Linking Undefined Behavior Undefined Behavior
All 10 Undefined Behavior questions