An extended region of the CORONA, exceptionally low in density and associated with unipolar photospheric regions.
ON-LINE GLOSSARY OF SOLAR-TERRESTRIAL TERMS of National Geophysics Data Center

An extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions having "open" magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and are a source of high-speed solar wind. Coronal holes are visible in several wavelengths, including solar x-rays and the He 1083 nm emission line.