From AMS Glossary
A pendant, curve-shaped region of reflectivity caused when precipitation is drawn into the cyclonic spiral of a mesocyclone.
The hook echo is a fairly shallow feature, typically extending only up to 3–4 km in height before becoming part of a bounded weak echo region (BWER).
Fujita, T. 1958. Mesoanalysis of the Illinois tornadoes of 9 April 1953. J. Meteor.. 15. 288–296.