INSTRUMENT (Lat. instrumenlum, from instruere, to build up, furnish, arrange, prepare), that which can be used as a means to an end, hence a mechanical contrivance, implement or tool; the word is more particularly applied to the implements of applied science, in mathematics, surgery, surveying, etc., while those of the handicrafts are generally known as " tools." A specific use of the term is for the various contrivances used to produce musical sounds, " musical instruments."
In law an " instrument " is any formal or written document by which expression is given to a legal act or agreement. This is a classical use of the Lat. inslrumentum, a document, record. The term may be used in a wide sense, as a mere writing, meant only to form a record, or in a particular sense with reference to certain statutes. For example, the Stamp Act 1891 defines an instrument as an expression including every written document; for the purposes of the Forgery Act 1861 a post-office telegram accepting a wager has been defined as an instrument. In expressions such as " deed, will, or other written instrument " the word means any written document under which a right or liability, legal or equitable, exists.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)