Русско-англо-русский юридический словарь

Furthermore, there is also a specific difference between a letter of intent (LOI) and a memorandum of understanding (MOU); an LOI outlines the intent of one party toward another with regard to an agreement, and may only be signed by the party expressing that intent, whereas an MOU must be signed by all parties to be a valid outline of an agreement. Nevertheless, LOIs are fairly often incorrectly referred to as MOUs and vice versa. (Wikipedia)

In Russian law, there is a concept of a preliminary agreement, which is an agreement to enter into a binding agreement in future on the terms described in the pleliminary agreement. Preliminary agreement has a binding effect on the parties. Therefore, there is a risk that a memorandum of understanding, letter of intent, or any other similar arrangements, if governed by Russian law, may be treated as "preliminary agreements" and, therefore, have a binding effect on the parties. (International Stock Purchase Acquisitions: Summaries of International Law, American Bar Association, 2006, Google Books)

A letter of intent (LOI or LoI, and sometimes capitalized as Letter of Intent in legal writing, but only when referring to a specific document under discussion) is a document outlining an agreement between two or more parties before the agreement is finalized. Such agreements may be Asset Purchase Agreements, Share Purchase Agreements, Joint-Venture Agreements, Lease Agreements, and overall all Agreements which aim at closing a financially large deal. (Wikipedia)

In commercial practice, particularly when transactions of considerable complexity are involved, it is quite frequent that after prolonged negotiations the parties sign an informal document called “Preliminary Agreement”, “Memorandum of Understanding”, “Letter of Intent��� or the like, containing the terms of the agreement so far reached, but at the same time state their intention to provide for the execution of a formal document at a later stage (“Subject to Contract”, “Formal Agreement to follow”). (UNIDROIT PRINCIPLES 2010)