A blade is a very sharp metal edge of a tool. A good cooking knife should have its blade sharpened regularly.
Knives, swords, and razors have blades, metal edges so fine and thin that they can cut and slice. The shoulder bone is sometimes called a "shoulder blade," and a piece of grass is also a blade. In fact, the original meaning of the Old English blæd was "leaf," and then "something resembling a leaf." In the 14th century, the word was applied to swords, based on their shape — similar to a leaf or blade of grass.