If you're like me - a busy tech worker with more commitments than time - you probably keep a list of things to do. Back in the day, a "to do" list, like all important scientific discoveries, had the terse prestige of a Latin sobriquet. The word agenda comes from the gerund of agere,
the Latin verb meaning "to do". It's an ellipsis, of sorts. Used in an expression such as
mihi agenda sunt ("things which are to be done by me"), the gerund is used to
express need or obligation. There are others still kicking around. Our memo is a truncation of memorandum, that which must be remembered. A referendum
is a decision which a government must refer to the people for their consideration.
Technically, agenda is a plural. If you are lucky enough to have only a lunch date with Cicero on your schedule, then you have an agendum. I can't find any references to the singular in the OED, except as agend, which doesn't seem to have survived the Age of Napoleon. In modern English we rely on the expression agenda items to evoke the individual components of a meeting's schedule. I like that expression, because "item" reinforces the notion that a meeting ought to progress, from start to finish, in an iterative manner. Too many meetings seem more like hallucinanda.
I suspect that the more recent action item was motivated by similarity of form to
agenda item (and not by their common etymology, "action" deriving from the past
participle of agere). We use agenda item to refer to the points we cover in discussion at a meeting, and action item to refer to the things that we have agreed to do after the meeting has ended. I'm not sure why we can't just call them actions. After all, we want to do things, and not just talk about them at meetings.
If anyone hasn't seen it yet, action item has inspired a singleton
comic strip, chock full of ludicrous corporate jargon.