Blood brother can refer to one of two things: two males related by birth, or two or more men not related by birth, who swear loyalty to one another. This is usually done in a ceremony, known as a Blood Oath, where the blood of each man is mingled together. In simple terms, this is an extension of fraternization.
Orvar-Odd's saga contains a notable account of blood brotherhood. The Norwegian warrior Orvar-Odd felt a desire to test his fighting skills with the renowned Swedish warrior Hjalmar. Thus Orvar-Odd sailed to Sweden with five ships and met Hjalmar who had fifteen ships. Hjalmar could not accept such an uneven balance of strength and sent away ten of his own ships so that the forces would be even. The two warriors fought for two days with a lot of blood-letting and poetry, but it was a draw. Finally, they realized that they were equals and decided to become sworn brothers by letting their blood flow under a strand of turf raised by a spear. Then the strand of turf was put back during oaths and incantations.
The Lydia ceremony involved nicking their arms with a sharp object and licking the blood off of each other's arms. The Scyths would allow their blood to drip into a glass where it was mixed with wine and drunk by both participants.
In Asian cultures, the act and ceremony of becoming "blood brothers" is generally seen as a tribal relationship, that is, to bring about alliance between tribes. It was practiced for this reason most notably among the Mongols and early Chinese. There is some evidence that Native Americans also did it for this purpose.
Blood brothers among large groups was common in ancient Mediterranean Europe where, for example, whole companies of Greek soldiers would become as one family. It was perhaps most prevalent in the Balkan Peninsula during the Ottoman era, as it helped the oppressed people to more effectively fight the enemy. Blood brothers were also common in Serbia, Albania and Bulgaria.
Blood brotherhood, highly ritualized and subjected to a strong code, was a common practice in the Caucasus, especially among the mountaineers. Some relics of this tradition survive to this day.
It is still practiced today, but mostly as a throw-back to tribal times. The tradition of intertwining arms and drinking wine in Greece and elsewhere, is believed to be a representation of becoming blood brothers.
In modern times, a common blood brother ceremony includes having each person make a small cut, usually on a finger or the forearm, and then the two cuts are pressed together and bound, the idea being that each person's blood now flows in the other participant's veins.