We can define an instrument as any object that is used for some purpose by the human being. The key to this definition lies in the fact that the ownership of the instrument is not a characteristic of the object itself, but is provided by the human being by conferring a functional use on it. (Strictly speaking, we could extend this definition to any animal species.

​

(Taken from http://www.eumus.edu.uy/docentes/maggiolo/acuapu/ins.html)