1911 Encyclopædia Britannica/Instrument
INSTRUMENT (Lat. instrumentum, 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, &c., 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. instrumentum, 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.