UML 1.1 Semantics Defines UML in terms of its abstract syntax presented in class diagrams, formal well-formedness rules expressed in OCL, and semantics expressed in text, by Rational.
UML 1.1 Summary An introduction to the UML that provides a road map to the other documents, by Rational.
UML Documentation Resources This series of documents defines the Unified Modeling Language (UML), a language for specifying, visualizing, and constructing the artifacts of software systems as well as for business modeling. The UML represents a collection of "best engineering practices" that have proven successful in the modeling of large and complex systems. (Rational)