architectural element that resembles the hollow upper half of a sphere

A dome is a building or roof that usually looks like the upper half of a sphere.

As a roof feature, domes make many religious and government buildings stand out, because if someone is speaking to others inside or under a dome, the voice sounds louder. Also a dome makes it easier to identify an important building, for example a temple or a palace.

A large stadium with a roof over the playing field is often called a "dome". One example of that usage is the Metrodome.