Chief altars in churches, typically raised on an elevated plane in the sanctuary where it may be seen simultaneously by all in the body of the church. In the early days of the Church the altar was usually placed in a chapel at the head of the edifice, the back of which looked directly towards the East and in such a way that it could be seen from any part by those in the church. When it was impossible to erect a church in such a manner, the altar was located opposite the chief doorway.

Source: AAT 

http://vocab.getty.edu/page/aat/300003728