Sharpness refers to the ability of an edge to cut or slice cleanly and efficiently. An edge is the line of intersection between two surfaces, and sharpening is the process of creating a very fine edge by polishing the two surfaces. There are several ways to test the sharpness of a knife or tool, including using it to cut through materials or shaving hair.
One simple test is to use the knife or tool to cut through vegetables or wood, observing how easily and cleanly it performs the task. Another test is to try to cut through a piece of paper using a slicing motion, or by pressing down on the edge of the paper. A dull knife will crumple the paper, while a sharp knife will cut it cleanly. Another test is to try to shave the hair on your arm - a dull knife will fold over the hairs without cutting them, while a sharp knife will cut them cleanly.
There are also more technical ways to measure sharpness, such as using a tool like the Edge-On-Up Sharpness Tester, which uses certified testing media to provide an objective measurement of sharpness. This can be useful for comparing the sharpness of different edges or for tracking the sharpness of a particular edge over time.
It's important to note that the angle of the edge can also affect sharpness, with lower angles producing sharper edges but also weaker edges that may require more frequent re-sharpening. Choosing the right level of sharpness will depend on the intended use of the knife or tool and the balance between sharpness and durability that is desired.