Agenda was originally the plural form of agendum, a Latin word meaning “something that needs to be done.” Hence, the plural agenda represented a list of items that needed to be dealt with.

Is there a plural of agenda?

"Although agenda is a plural word, it is pedantry to object to the common and convenient practice of thus treating it as a singular one. If a singular is needed for one item of the agenda there seems no escape from the rather cumbrous phrase; agendum is pedantic and agend obsolete."

How do you use the word Agendum?

Agenda, “things to be done,” is the plural of the Latin gerund agendum and is used today in the sense “a plan or list of matters to be acted upon.” In that sense it is treated as a singular noun; its plural is usually agendas: The agenda is ready for distribution.