It’s a curved sword that’s light enough to dual-world. Beyond that, the description is pretty much up to the player or DM.
I know this was a typo, but I love it! The sword can be a cutlass or a scimitar depending on what world you're in.
The longsword is mainly 1400s, while slightly earlier and slightly later.
But there are precocious examples of it from the 1100s on. These medieval specimens are normal swords, but experimentally made longer to suit the preference of certain individuals.
In other words, these are "long swords", but not yet "longswords".
Similarly, there might medieval "falchions" that are precocious examples of cutlasses, that happen to be small and thin to suit the preference of certain individuals. These might even be gracile enough to function as an offhand weapon, even if they were never used this way. Even among the later modern cutlasses, only the smaller thinner ones would be useful as an offhand weapon.
So the flavor of a 5e "scimitar" can be medieval weirdness. Where the viewers perceive it as falchion but notice its wielder has a strange version of it.
My impression is, in Europe, two-weapon fighting is modern, and evolving from Renaissance streetfighting. As far as I know, there are no medieval mentions of it. But the modern styles show it is possible to use medieval weapons for two-weapon styles.
Note, it is possible to fight with two rapiers, two katana, two viking-knightly swords. These are not normally offhand weapons. They are agile ("finesse") but not offhand ("light"). Those individuals who use two of them are remarkable, and it is more like a D&D feat than a weapon property.