noun
highest-ranked wrestler in a stable
Sumo term: the top-ranked wrestler within a heya (stable), responsible for discipline and guidance of junior wrestlers.
彼は部屋頭として、後輩の指導にあたっている。
As the highest-ranked wrestler in the stable, he is responsible for coaching the junior wrestlers.
横綱 is the highest rank in sumo overall, while 部屋頭 is the highest-ranked wrestler within a particular stable, who may or may not be a yokozuna.
Compound of 部屋 (heya, 'stable') and 頭 (kashira, 'head' or 'leader').