Keeping in mind the latest state of the art PCB technology, we can define the HDI PCBs as those printed circuit boards that use some or all of these features: microvias, blind and buried vias/microvias techniques, built-up PCB laminations and high signal performance considerations.
One of the key driving factors is to achieve more connections per area of the Printed Circuit Board. This results in a more compact, dense, and lower layer count PCBs.