UML and other related modelling languages are exists in most of the system engineering fields to represent the system, flow, relations in a structured way. UML also is one of the modelling language used in computer science like other industries to represent the systems in obeject oriented way by using different types of diagrams. Does 'Unified' UML has a special (or real) meaning here?
There is a history behind UML that it combines the object oriented methodologies advocated by three camps. James Rumbaugh had a group that created an object oriented methodology called Object Modeling Technique (OMT), Grady Booch had overlapping work and notation that I always used to just call the Booch method, and Ivar Jacobson had a methodology based on use cases.
The unification in unified, is the unification of their methodologies.
Another piece of trivia is that they called themselves the three amigos (friends) and were part of Rational before it was sold to IBM.