(ISP) I think "data" can be both but I agree that to insist that it is always plural is pretentious and a bit annoying, a bit like government propagandums. (Phil) Don't quite remember 'em, but if you say so. :-)
[Phil] Thanks. I'm not overly conversant with WWII UK memes unfortunately. In fact, Mr. Chad (known elsewhere as Kilroy) is the sum total of my knowledge of the matter. Or was, since I now know something else thanks to you and FGZStar.
I recognised the reference to 'Keep Calm and Carry On' - available on mugs near you - but still don't see what FGZstar means. I'm happy to accept that it's just a generational thing.
Datum and agendum are singular. One singular piece of data and one singular thing to do, rather than a list. The minute it is two (or more) they become data and agenda. So, "What's on the agenda?" is correct being people are going to expect more than one item. If I say the only thing on the agenda is the meeting, that's wrong, it's the agendum. Yes, I am just being a grammar nazi. I don't really care that much about those words.