Legal Definition of Instrument: A written document, such as a deed, a mortgage, a negotiable security, a contract, a will, etcetera | Discover the Legal Definition of an Instrument and its Importance in Business Transactions. Learn about deeds, mortgages, negotiable securities, contracts, wills, and more. Understand how instruments provide legal validity, clarity, protection, compliance, and evidence in disputes. Ensure business continuity and protect your interests with a solid understanding of instruments in the world of business