A semidome is a half of a dome, like a round roof that is cut in half.
A semidome can make people think of big, old buildings or special rooms.
Examples:
The church has a semidome over the altar.
The semidome is painted with stars and angels.
I like to look at the semidome when I visit the church.
The semidome looks like a big smile in the sky.
In the old castle, there is a room with a semidome.
The semidome is the best part of the whole building.