A notcher is a surrounded group of stones on the side, consisting of stones on the third line, flanked by stones on the second line.
Example of a notcher. If it weren't for the "notch" at the circled point and the marked stones, all Black stones would be on the second line.
The discussion of the life and death status of notchers and their classification have been separated into
On all these pages you will find a code name for each particular notcher. See notcher code for an explanation.