Ben Zimmer forwarded me this post from Threads, in which the writer Ruth Ware finds a transatlantic difference between AmE and BrE interpretations of oblong.

(If you don't know what she means by "'frown' all over again", see this old post!)

Where oblong started

Oblong derives from Latin oblongus, meaning 'of greater length than breadth, elongated' (OED). So, you can already see that what Ware is calling the US meaning is in tune with the etymology. 

The Oxford English Dictionary's definition indicates (with 'esp.') that it has come to specifically refer to rectangles. It's a word whose meaning has narrowed with use. 

1.a.

?a1425–

Elongated (usually as a deviation from an exact square or circular form); esp. rectangular with the adjacent sides unequal.

The more particular interpretation, then, is oblong = non-square rectangle. That seems a bit redundant from my American perspective because everyone who knows the word oblong probably already knows the word rectangle, and only in geometry classes is a square a rectangle. But we'll be back at geometry class soon enough. 

Are AmE & BrE oblong really different?

Is this a transatlantic linguistic difference?  It seems so—though American dictionaries seem to show more awareness of the 'rectangle' meaning, more than UK dictionaries show about the 'elongated shape' meaning. This seems to suggest that the shift in meaning has quickly taken over in BrE and but remains incomplete in AmE. Here come a lot of dictionary entries.

The Cambridge Dictionary (a learner-orient[at]ed dictionary) is the only dictionary I've found that marks it as a US/UK difference—and a very firm one at that.

Cambridge Dictionary

Another British [learner] dictionary, Collins COBUILD, just defines the 'rectangle' meaning, as does the version of the Oxford Dictionary of English (not the same as the Oxford English Dictionary) that came with my Mac.

But the two American dictionaries on the Collins dictionary site give both meanings, without marking one as British:

And Merriam-Webster has just the older meaning:

So when did AmE & BrE oblong split?

The fact that the etymological meaning is the current meaning in AmE signals that this is a change that happened in the UK (or at least outside the US!) after AmE and BrE split. 

She was replying to someone else who'd said: "Children see the shape represented as a rectangle everywhere, yet it is actually an oblong that is part of the ‘rectangle family’."  

Is it just oblongs?

Ware's mention of ovals and ellipses made me think: do I know the difference between those? The website Meaningful Math (and that lack of 's' is meaningful) says:

...which is to say that for that site, as for me, an oval can be symmetrical horizontally and vertically (an ellipse) or tapered (like an egg shape). But there are plenty of other sites willing to tell you that an oval is egg-shaped, in contrast to an ellipse—and those, as far as I can see, seem to have to do with the math(s) curriculum in India. But Ware seems not to count ellipses as ovals—so does oval = egg-shape have currency in Britain? 

The Oval, the cricket ground in London, looks like neither an egg nor an ellipse!