spear or pilum?

[D B Campbell]

Synomys are used for varying weapons, and clearly the names of weapons over time become used for entirely different weapons.

Well said, Robert!

I just noticed this thread -- apologies for chipping in so belatedly -- and my first thought was the terminology of Arrian (a "technical" writer whose terminology can be infuriatingly obscure). You have gone one better with your excellent catalogue of examples!

The papyri and original documents ought to be the safest evidence, reflecting actual contemporary usage. But of course, the fact that the cavalry troopers at Carlisle were using lanceae doesn't necessarily mean that all cavalry troopers used this weapon.