Real Dictionary

tool
countable noun: A tool is any instrument or simple piece of equipment that you hold in your hands and use to do a particular kind of work. For example, spades, hammers, and knives are all tools.
countable noun: If you describe someone as a tool of a particular person, group, or system, you mean that they are controlled and used by that person, group, or system, especially to do unpleasant or dishonest things.
countable noun: You can refer to anything that you use for a particular purpose as a particular type of tool.

tool in American English
any implement, instrument, or utensil held in the hand and used to form, shape, fasten, add to, take away from, or otherwise change something by cutting, hitting, digging, rubbing, etc.: knives, saws, hammers, shovels, rakes, etc. are tools
intransitive verb: to work with a tool
noun: an implement, esp. one held in the hand, as a hammer, saw, or file, for performing or facilitating mechanical operations
transitive verb: to work or shape with a tool

tool in British English
noun: an implement, such as a hammer, saw, or spade, that is used by hand

tool in Mechanical Engineering
noun: A tool is a device, usually a simple one, for making or assembling components or parts.