A sweetheart agreement is typically an informal agreement between two parties. It can also be known by various other names including a handshake agreement, a gentlemen's agreement, a private agreement, or a verbal agreement. Essentially, it means that both parties have agreed to something but have not put it in writing or made it official. This type of agreement can be problematic as it opens the door for misunderstandings or disagreements later on. Other synonyms for a sweetheart agreement include an under-the-table deal, an unwritten pact, and a private arrangement. It's always recommended to have a formal agreement in writing to avoid any future discrepancies.