A circular letter is a document or letter that is sent to a closed group of people with the intention of being widely circulated. In business, circular letters are used to inform their recipients of new policies or other important matters. They might be sent to all of a company's employees or clients to notify them of new company plans or a change of address.
Circular letters have also been used in politics. Samuel Adams famously wrote a missive about the unconstitutionality of the Townshend Acts in 1768. He expressed the opinion that the taxes that the British Parliament was levying against the colonies were unjust because of the lack of colonial representation in Parliament. It was passed by the Massachusetts House of Representatives, and became known as the Massachusetts Circular Letter.