Definition:
VHDL is the VHSIC Hardware Description Language. VHSIC is an abbreviation for Very High Speed Integrated Circuit. It can describe the behaviour and structure of electronic systems, but is particularly suited as a language to describe the structure and behaviour of digital electronic hardware designs, such as ASICs and FPGAs as well as conventional digital circuits.
VHDL is a notation, and is precisely and completely defined by the Language Reference Manual ( LRM ). This sets VHDL apart from other hardware description languages, which are to some extent defined in an ad hoc way by the behaviour of tools that use them. VHDL is an international standard, regulated by the IEEE. The definition of the language is non-proprietary.
VHDL is not an information model, a database schema, a simulator, a toolset or a methodology! However, a methodology and a toolset are essential for the effective use of VHDL.
Further Reading:
Book: The Unified modeling Language user guide by Grady Booch, James Rumbaugh and Ivar Jacobson