The Full Form of LOI is Letter of intent.

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 the understanding between two or more parties which understanding they intend to formalize in a legally binding agreement. The concept is similar to a heads of agreement, term sheet or memorandum of understanding. Such outlined agreements may be merger and acquisition transaction agreements, joint venture agreements, real property lease agreements and several other categories of agreements that may govern material transactions.