I've also seen "data point" used when a singular form is required. I think in common usage, "data" is a mass noun like "mayonnaise" rather than a plural.

That's a good point! Because the compound noun rules in English normally call for all nouns, other than the head of the noun phrase, to be singular.

This applies to the singulars that denote single objects too: scissor lift, pant leg; not scissors lift or pants leg.

According to this principle, it should be datum point, where only the head noun point may be pluralized.

"Datum point" exists, but I've never seen or heard it in use; it's just a "confirm by Google".

> in common usage, "data" is a mass noun like "mayonnaise"

Were you also commenting on its pronunciation? I'd instead say: "data" is a mass noun like "mahyonnaise" and "tomahto sauce".

'Data point' - much better!

"Datum" (and its plural, "datums") is legitimate if you're talking about, eg, one filled-out survey form, or one array element. It's basically like "water molecule"; you wouldn't say "these water are cold", but "this datum is a outlier" makes sense.

While I understand, and agree that you are correct, I still don't like this wording. I prefer the sound of 'this data point is an outlier' or 'this point is an outlier'

> that ship has sailed

Wha? Oh, you mean quae navis navigaverunt.

Haha yes of course!