A dome is an architectural element consisting of the hollow upper half of a sphere, or of a shape close to this; the dome may be elliptical, stretching upwards or shallow rather than spherical.
Although there are earlier examples of dome, its breakthrough came with the Roman invention of concrete. One of the most famous Roman domes is that of the Pantheon in Rome, now the church Santa Maria ad Martyres.
The largest domes in Roman churches are, in descending order, those of San Pietro in Vaticano, Sant'Andrea della Valle and San Carlo ai Catinari.
On top if the dome there is usually a lantern, a tower-like structure, so called because it has openings that give light into the church.
The inside of the dome is generally decorated, with everything from a simple coffered ceiling til elaborate frescoes or mosaics.