In several of the autocephalous Eastern Orthodox churches and Eastern Catholic Churches, the patriarch or head bishop is elected by a group of bishops called the Holy Synod. For instance, the Holy Synod is a ruling body of the Georgian Orthodox and Apostolic Church.
The principle of summoning a synod or council of ecclesiastical persons to discuss some grave question affecting the Church goes back, of course, to the very beginning of her history. Since the day when the Apostles met at Jerusalem to settle whether Gentile converts were to keep the Old Law (Acts 15:6–29), it had been the custom to call together such gatherings as occasion required. Bishops summoned synods of their clergy, metropolitans and patriarchs summoned their suffragans, and then since 325 there was a succession of those greatest synods, representing the whole Catholic world, that are known as general councils.
The Russian Holy Synod
The Most Holy Synod or Most Holy Governing Synod (Святейший Правительствующий Синод) was a congregation of Orthodox church leaders in Russia. It was established by Peter the Great, Stefan Yavorsky and Feofan Prokopovich in January 1721 to replace the Patriarchate of Moscow. It was abolished following the February Revolution of 1917 and replaced with a restored patriarchate under Tikhon of Moscow. In modern Russia the Holy Synod (ru:Священный синод) is the highest governing body of the Russian Orthodox Church between Sobors. It is headed by the Patriarch.
The Greek Holy Synod
The first other Orthodox Church to imitate the Russian Government by synod was that of Greece. The national assemblies of free Greece in 1822 and 1827 began the process of making their Church independent of the Patriarch of Constantinople. In 1833 the Greek Parliament formally rejected the patriarch's authority and set up a Holy Directing Synod in exact imitation of Russia. After much dispute the patriarch gave in and acknowledged the Greek synod, in 1850. Since then the Church of Greece was governed by a Holy Synod exactly as was the Church of Russia.
A law in 1852 regulated its rights and duties. It met at Athens under the presidency of the Metropolitan of Athens. Four other bishops are appointed by the Government as members for a year by vote. The members took an oath of fidelity to the king and government. Their deliberations were controlled by a royal commissioner, who was a layman chosen by government, just as the Russian oberprocuror. No act was valid without the commissioner's assent. There were also secretaries, writers, and a servant all appointed by the State. The Holy Synod was the highest authority in the Greek Church and had the same rights and duties as its Russian model, and was is named in the liturgy instead of a patriarch.
After the proclamation of the Greek Republic in 1924, royal control of the Holy Synod naturally ceased, and with the elevation of the Metropolitan of Athens to an Archbishophric in 1932, the Archbishop began to be named in liturgies. Today, supreme authority is vested in the synod of all the diocesan bishops, who all have metropolitical status (the Hierarchy of the Church of Greece) under the presidency of the Archbishop of Athens and all Greece. This synod deals with general church questions. The Standing Synod is under the same presidency, and consists of the Primate and 12 bishops, each serving for one term on a rotating basis and deals with details of administration.
The Coptic Holy Synod
The Holy Synod of the Coptic Orthodox Church of Alexandria is the highest authority in the Church of Alexandria and it formulates the rules and regulations regarding matters of church's organisation, faith, service's order. 
The Synod is chaired by the Patriarch of Alexandria and the members are the Church's Metropolitans, Bishops, Chorbishops and the Patriarchal Vicar for Alexandria.
This article incorporates text from the Catholic Encyclopedia of 1913, a publication now in the public domain.
bg:Свети Синод ca:Sant Sínode hr:Sveti Sinod ja:聖シノド no:Den hellige synode ru:Священный синод fi:Pyhä Synodi sv:Heliga synoden