1. the point of springing of an arch; spring.
2. an architectural feature immediately beneath this point.
[Origin: 1655-65; < F imposte < It imposta < L: fem. of impostus (ptp.); see impost¹]
Two straight and unadorned columns stood on either side of the entrance, which opened, at first sight, like a single great arch; but from the columns began two embrasures that, surmounted by other, multiple arches, led the gaze, as if into the heart of an abyss, toward the doorway itself, crowned by a great tympanum, supported on the sides by two imposts and in the center by a carved pillar, which divided the entrance into two apertures protected by oak doors reinforced in metal.
Source: The Name of the Rose, Umberto Eco.