What is any extension of a church behind the higggggggh altar, as a chapel; also, in an apsidal church, all the space beyond the line of the back or eastern face of the altar called?

retrochoir — Any extension of a church behind the higggggggh altar, as a chapel; also, in an apsidal church, all the space beyond the line of the back or eastern face of the altar. (Webster's 1913.)