An article by Tom Hill, Intel published in ElectronicDesign
An HDL is used to implement a register-transfer-level (RTL) abstraction of a design. In time, the process of creating RTL abstractions was made easier through the use of reusable IP blocks, speeding the design-flow process. As designs became more complex and the time-to-market pressures increased, developers and the vendor community have strived to provide more software-based tool chains to help reduce development times.
One of these techniques is “high level synthesis” (HLS). HLS can be thought of as a productivity tool for hardware designs. It typically uses C/C++ source files to generate RTL that is, in most cases, optimized for a particular target ..