Null

Definitions

1. A value denoting the deliberate absence of a value, and the source of one of computing's most expensive design mistakes. Tony Hoare introduced the null reference to ALGOL W in 1965 and later called it his billion-dollar mistake, on the grounds that it makes every reference potentially invalid while the type system claims otherwise. The result is the null pointer dereference, a failure mode that has produced innumerable crashes and security vulnerabilities across six decades. Modern languages respond either by eliminating null in favour of option types, as in Rust and Haskell, or by tracking nullability in the type system, as in Kotlin, Swift, and later versions of C#.