A thing to be done.
Yes, you guessed it. This is the singular of agenda. And yes, its use is somewhat archaic. OED calls it «now rare» and Chambers doesn’t list it at all. Agenda’s modern usage is clearly singular; one looks at the agenda before beginning a meeting, and someone promoting political views is said to have an agenda.
But it is at least useful to understand agenda as a list, as opposed to simply «that which someone wants done». It’s a list, a set. And this is why it is grating (though not incorrect) to hear references to «agendas», especially when these agendas belong to a single person.