It was developed by the Programming Research Group at Oxford University in the late 1970s and is based on rather standard mathematical notation used in axiomatic set theory, lambda calculus, and first-order predicate logic. All expressions in Z notation are typed, thereby avoiding some paradoxes of naive set theory. It contains a standardized catalog (the "mathematical toolkit") of commonly used mathematical functions and predicates.
The ISO completed a Z standardization effort in 2002. This standard, entitled Information Technology - Z Formal Specification Notation - Syntax, Type System and Semantics, ISO/IEC 13568:2002, can be bought directly from ISO.
See also: Z++