The half-dome over the apse of a Byzantine church is commonly called a conch.
In architectural language, a conch is a semi-dome covering an apse, the usually curved termination of a church sanctuary. Byzantine builders frequently used the conch as a broad surface for images of Christ, the Virgin Mary, saints, or liturgical scenes.
The term comes from the Latin concha, meaning shell, reflecting the curved form. In Byzantine churches, the apse and its conch often formed a visual focus behind the altar. The shape also helped transition from the vertical wall of the apse to its curved ceiling.
A conch is not the same as a pendentive. Pendentives are curved triangular surfaces that support a dome over a square or polygonal space. A narthex is an entrance hall, while an ambulatory is a passage surrounding a central space or apse.