- Agenda and agendum are Latin words. An agendum (see -um in Latin) is 'a thing which should be done', 'an item which should be dealt with during a meeting'. The regular plural in the Latin language for nouns ending in -um is -a. An agendum is a single item of business to be transacted during a meeting, all of whose business will be listed on an agenda.
- In current British English, the word Agenda is only used as 'a list of things to be done'. This, for us, is nearly always the list of topics to be discussed at a meeting. (In what I consider to be best practice for those who run meetings, an agenda is not more than a page in length.) In some other languages, a pocket-book with lists of 'things to be done' each day - meetings, jobs, appointments, etc - is called an 'agenda'. In Britain, this is a diary - sometimes an appointments diary, to distinguish it from the record of one's life that shares the name of diary, but is also called a journal. The appointments diary looks forward, to what is going to happen (or is intended to happen). The journal or diary looks back. It records and reflects on what has happened.
- Attentive readers will note that the ending -a indicates 'things' - it is a plural ending. So agenda is a plural. Those who like formal English like to remember that the singular form ought to be agendum. "'There is one agendum that causes concern,' said the Vice-Chancellor. 'I dislike the whole agenda,' said the Professor."
There are many, even in academic life, who regard this kind of nit-picking, based on a knowledge of classical language, as unnecessary, and a waste of time. As usual, AWE would advise you to do what your teachers prefer. If you don't know what they prefer, do as you like - and if what you like is to sound like a potential academic, use agendum for the singular!
Those who know no Latin often say "an agenda item", or "an item on the agenda", as the 'singular' ~ 'one topic to be discussed at the meeting'.