from The Century Dictionary and Cyclopedia
- n. An impost taking the form of a separate member with a definite character, as in Byzantine and early Italian architecture where it rises like a separate capital above the capital proper of a column. The term is also used for the two blocks from which an arch in a continuous wall seems to arise and upon which it bears.
Sorry, no etymologies found.
The impost-block was almost universal, except where its use was rendered unnecessary by giving to the capital itself the massive pyramidal form required to receive properly the spring of the arch or vault.