Diagonal pliers (or wire cutters or diagonal cutting pliers or diagonal cutters) are pliers intended for the cutting of wire (they are generally not used to grab or turn anything). The plane defined by the cutting edges of the jaws intersects the joint rivet at an angle or "on a diagonal", hence the name.
Instead of using a shearing action as with scissors, diagonal pliers cut by indenting and wedging the wire apart. The jaw edges are ground to a symmetrical "V" shape, thus the two jaws can be visualized to form the letter "X", as seen end-on when fully occluded. The pliers are made of tempered steel, and inductive heating and quenching are often used to selectively harden the jaws.