Hardware Description Language(HDL)

A Hardware Description Language HDL) is a software programming language used to describe the operation of a piece of hardware. VHDL and Verilog HDL are the high level description languages for system and circuit design. These languages support various abstraction levels of design, including architecture-specific design. At the higher levels, these languages can be used for system design without regard to a specific technology. Efficient, standard HDL code is essential for creating good designs. The structure of the design is a direct result of the structure of the HDL code. Additionally, standard HDL code allows designs to be reused in other designs or by other HDL designers.
Top-down, HDL-based system design is most useful in large projects, where several designers or teams of designers are working concurrently. HDLs provide structured development. After major architectural decisions have been made and major components and their connections have been identified, work can proceed independently on subprojects.

HDLs: VHDL, Verilog, AHDL, etc.....