Definition of 'Letter Security'
A security that is not registered with the SEC, and therefore cannot be sold publicly in the marketplace. Letter securities are sold directly by the issuer to the investor. The term is derived from the SEC requirement for an "investment letter" from the purchaser, stating that the purchase is for investment purposes and is not intended for resale.
Also known as restricted security, letter stock or letter bond.
Investopedia explains 'Letter Security'
Letter securities are often offered by means of private placements to institutional investors. They may also be offered to accredited investors, who may need to demonstrate that they possess the financial means to invest in, and the expertise to assess the risks of, such securities. Letter securities may have conditions attached to them such as a mandatory hold period. They are sometimes registered with the SEC at a subsequent date.