A plural term for brothers, either as siblings or Christian brothers. "Who is My mother and who are My brothers? And He stretched out His hand toward His disciples and said, Here are My mother and My brothers!" (Matt. 12:48-49).

(plural) the lay members of a male religious order

n. pl. Members of a brotherhood, gild, profession, association, or the like.

All male members of the Church; "the Brethren," a designation of the General Authorities of the Church.

Brethren is a plural of brother, although it is rarely used. It is in principle synonymous with "brothers". The term is mainly used within Christianity, either in (rare) reference to a fraternal order or fraternal military order, or to refer to any of several Christian groups/denominations, most of which are Anabaptist-Pietist.