Hard steel, usually crucible steel, capable of being tempered so as to be suitable for tools.
This term covers a wide range of high-carbon and alloy steels used to make various types of tools. Tool steels are easy to machine before heat-treating but get very hard after that. Grinding and lapping are the only ways to machine tool steels after heat-treating. Common classifications of tool steels are A steels (air hardening) and O steels (oil hardening)
A generic term covering a whole range of both carbon and alloy steels commonly used to make tools. This therefore includes steels used to make cutting tools, press tools, dies, moulds for both plastics and die-casting, hand-tools, in fact any tool used to shape metals or other materials.