What area or object in an Eastern Christian church is the “iconostasis”?

The iconostasis, (Greek: "εἰκονοστάσιον") in Eastern Christian churches of Byzantine tradition, is the solid screen of stone, wood, or metal, usually separating the sanctuary from the nave.

Sanctuary, in any church, is the sacred place set apart from the profane, ordinary world where priests, or other religious people, have access. Nave, is the central and principal part of a Christian church, extending from the entrance (the narthex) to the transepts (transverse aisle crossing the nave in front of the sanctuary in a cruciform church) or, in the absence of transepts, to the chancel (the area around the altar).

The nave is that part of a church set apart for the laity, as distinguished from the chancel, choir, and presbytery, which are reserved for the choir and clergy. The separation of the two areas may be effected by screens or parapets, called “cancelli”.

The iconostasis had originally been some sort of simple partition between the altar and the congregation; it then became a row of columns, and the spaces between them were eventually filled with icons. In later churches, it has extended to the width of the sanctuary, though the height may vary, and is covered with panel icons.