师兄 is a term used in traditional Chinese master-apprentice relationships (e.g., martial arts, academic, or craft lineages) to refer to a male senior fellow apprentice or junior master who started training or studying before you. The relationship implies hierarchy and mutual respect — you address or refer to them as 师兄 regardless of actual age, based solely on the order of apprenticeship. This term is also used for the master's biological son if he is older than you, but the more common usage is for non-blood-related senior peers.