tym·pa·num /ˈtɪmpənəm/
-noun, pluralnums, –na /-nə/
1. Anatomy, Zoology.
a. middle ear.
b. tympanic membrane.
2. Architecture.
a. the recessed, usually triangular space enclosed between the horizontal and sloping cornices of a pediment, often decorated with sculpture.
b. a similar space between an arch and the horizontal head of a door or window below.
3. Electricity. the diaphragm of a telephone.
4. a drum or similar instrument.
5. the stretched membrane forming a drumhead.

[Origin: 1610-20; < L < Gk týmpanon drum, akin to týptein to beat, strike]

Source: Dictionary.com.

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.