abbot; archimandrite
Title for the head of a large monastery, especially in Christian contexts. 修道院長 is a more general term for abbot.
He was appointed abbot of the monastery.
修道院長 is a general term for abbot, while 大修道院長 specifically refers to the head of a larger or more important monastery, often translated as archimandrite.
Compound of 大 (dai, 'great') + 修道院 (shūdōin, 'monastery') + 長 (chō, 'head'). The term is used in Christian contexts for the superior of a major monastery.