generally defined as 3mhz to 30mhz.
What makes it interesting here is that it's a frequency range that is commonly-used and that also propagates in modes other than line-of-sight. For instance 'groundwave' propagation curves around the Earth because of induced currents in the Earth.
This allows HF communication over the horizon. Enthusiasts are regularly able to make contact between continents with amplifier power of less than 5 Watts.
For most of the HF spectrum reflections off the ionosphere are the main useful propagation mode. At the short end (like CB radio) bouncing of the ionosphere is harder so line-of-sight is more common.