letter of engagement
From Longman Business Dictionaryletter of engagementˌletter of enˈgagement noun (plural letters of engagement) [countable] a letter in which the conditions under which someone is employed are stated. In the US, a letter of engagement is usually given to a CONTRACTOR who has been employed to carry out a particular service or jobSYNENGAGEMENT LETTERIf you deviate from the conditions laid down in this letter of engagement, we will be free to terminate the engagement at any time.