【Mandate】 ,  【initiative】 ,  【referendum】 ,  【plebiscite】  are comparable when meaning a political action or procedure whereby a constituency instructs or gives information of its desires to its legislature or legislators.

【Mandate】 , the most general of these terms, applies to instruction delivered by the people (as by a general vote or by a choice in an election) that makes their wishes clear not only to their representatives in a legislature but also to those who hold the executive power, or, in an extended sense, to those who represent them in any way or who by the nature of their office or duties are necessarily responsive to the will of the people.

【Initiative】 often denotes a right, but when it denotes a procedure, it implies recognition of the fight of a group of voters or, more often, of a clearly defined number of voters, to propose a new measure or a constitutional amendment to a legislature.

【Referendum】  applies to the practice, adopted by some states and cities in the United States, of sending measures that have been considered by or proposed to the legislative body to the voters for approval or rejection or for an expression of their wishes.

【Plebiscite】  basically applies to a vote of the people usually by universal suffrage on some measure submitted to them by the group or the body having the 【initiative】 . Sometimes  【plebiscite】  implies a vote of the population of a territorial unit that testifies to their wishes especially as to the form of government they will accept, or their choice in a proposed merger with either of two nations.