What it says on the tin - what is the name of the post that holds the classical set of scales called? I keep wanting to call it the fulcrum, but I’m pretty sure that can’t be it.
You may hear pivot or knife-edge.
Knife edge would be a description of the best simple configuration of the top of the fulcrum as a whole. Wiki’s interesting entry, “weighing scales,” refers to them as fulcrums, and for more precise balances, “…for highest accuracy, the fulcrum would consist of a sharp V-shaped pivot seated in a shallower V-shaped bearing.”