Traditional Case pocket knives
A nail nick is a groove in a folding knife's blade that helps the user open it. Kershaw defines it as "a groove in the top of the blade, usually halfway down the length of the blade" that "is found on manual folders." Böker, which also calls it a "nail groove," says the notch "gives the thumbnail hold when opening the blade."
The nick's shape varies by knife and its name by maker. Böker says that "In traditional American pocket knives, the nail nick is occasionally executed as a long groove parallel to the back of the blade." Buck's word is nail notch: pocket knives, Buck says, are typically "two-hand open/close and have nail notches to aid opening." Kershaw's steps for opening a manual folder say to "hold the blade at the nail nick" with the other hand, and the nick's neighbors on a folder are set out in what the parts of a pocket knife are called.
Kershaw's point that the nick is found on manual folders covers locking knives as well: "The blade on most manual folders will lock into position," Kershaw says, and "A slipjoint model will not have a locking mechanism." The groove therefore says nothing about the lock, and a slip joint is the folder defined by having none.
Böker calls the nick "part of the standard equipment of two-handed knives today," and Buck gives one-hand opening to the thumb stud or hole: "A blade thumb stud or hole enables one-hand opening."