Synod

A synod is a council of a Christian denomination, usually convened to decide an issue of doctrine, administration or application. The word synod comes from the Ancient Greek σύνοδος ; the term is analogous with the Latin word concilium. Originally, synods were meetings of bishops, and the word is still used in that sense in Catholicism, Oriental Orthodoxy and Eastern Orthodoxy. In modern usage, the word often refers to the governing body of a particular church, whether its members are meeting or not. The term is also sometimes used to refer to a church that is governed by a synod.
Sometimes the phrase "general synod" or "general council" refers to an ecumenical council. The word synod also refers to the standing council of high-ranking bishops governing some of the autocephalous Eastern Orthodox and Oriental Orthodox churches. Similarly, the day-to-day governance of patriarchal and major archiepiscopal Eastern Catholic Churches is entrusted to a permanent synod.

Usages in different Communions

Eastern Orthodox and Oriental Orthodox

In Eastern Orthodox and Oriental Orthodox churches, synods of bishops are meetings of bishops within each autonomous Church and are the primary vehicle for the election of bishops and the establishment of inter-diocesan ecclesiastical laws.
A sobor is a formal gathering or council of bishops together with other clerical and lay delegates representing the church to deal with matters of faith, morality, rite, and canonical and cultural life. The synod in the Western churches is similar, but it is distinguished by being usually limited to an assembly of bishops.
The term is found among those Eastern Orthodox Churches that use Slavic languages, along with the Romanian Orthodox Church.

Assembly

The presence of clerical and lay delegates is for the purpose of discerning the consensus of the church on important matters; however, the bishops form an upper house of the sobor, and the laity cannot overrule their decisions.
Kievan Rus' chronicles record the first known East Slavic church sobor as having taken place in Kiev in 1051. Sobors were convened periodically from then on; one notable assembly held in 1415 formed a separate metropoly for the church in the Grand Duchy of Lithuanian lands.
Important sobors in the history of the Russian Orthodox Church are:

Vladimir's Sobor in 1276
The Stoglavy Sobor in 1551
The Moscow Sobor of 1666–1667, to deal with disputes surrounding the ecclesiastical reforms of Patriarch Nikon
The All-Russian Sobor of 1917, which restored the Moscow Patriarchate and elected Saint Tikhon as the first modern Patriarch of Moscow
The All-Russian Sobor of 1988, called on the 1000th anniversary of the Baptism of Rus' to guide the church in the wake of glasnost and the loosening of the Soviet grip over the church

A bishop may also call a sobor for his diocese, which again would have delegates from the clergy, monasteries and parishes of his diocese, to discuss important matters. Such diocesan sobors may be held annually or only occasionally.

Roman Catholic

In Roman Catholic usage, synod and council are theoretically synonymous as they are of Greek and Latin origins, respectively, both meaning an authoritative meeting of bishops for the purpose of church administration in the areas of teaching or governance. However, in modern use, synod and council are applied to specific categories of such meetings and so do not really overlap. A synod generally meets every three years and is thus designated an "Ordinary General Assembly". However, "Extraordinary" synods can be called to deal with specific situations. There are also "Special" synods for the Church in a specific geographic area such as the one held November 16 – December 12, 1997, for the Church in America.

Synod of Bishops

While the words "synod" and "council" usually refer to a transitory meeting, the term "Synod of Bishops" or "Synod of the Bishops", is also applied to a permanent body established in 1965 as an advisory body of the pope. It holds assemblies at which bishops and religious superiors, elected by bishops conferences or the Union of Superiors General or appointed by the Pope vote on proposals to present for the pope's consideration, and which in practice the pope uses as the basis of "post-synodal apostolic exhortations" on the themes discussed. While an assembly of the Synod of Bishops thus expresses its collective wishes, it does not issue decrees, unless in certain cases the pope authorizes it to do so, and even then an assembly's decision requires ratification by the pope. The pope serves as president of an assembly or appoints the president, determines the agenda, and summons, suspends, and dissolves the assembly.
Modern Catholic synod themes:

X "The Bishop: Servant of the Gospel of JESUS CHRIST for the hope of the world" 1998
XI "The Eucharist: Source and Summit of the Life and Mission of the Church" 2005
XII "The Word of God in the Life and Mission of the Church" 2008
XIII "New Evangelisation for the Transmission of the Christian Faith" 2012
Extraordinary General "The Pastoral Challenges of the Family in the Context of Evangelization" 2014
Councils

Meetings of bishops in the Roman empire are known from the mid-third century and already numbered twenty by the time of the First Council of Nicaea. Thereafter they continued by the hundreds into the sixth century. Those authorized by an emperor and often attended by him came to be called ecumenical, meaning throughout the world. Today, Council in Roman Catholic canon law typically refers to an irregular meeting of the entire episcopate of a nation, region, or the world for the purpose of legislation with binding force. Those contemplated in canon law are the following:

An ecumenical council is an irregular meeting of the entire episcopate in communion with the pope and is, along with the pope, the highest legislative authority of the universal Church. The pope alone has the right to convoke, suspend, and dissolve an ecumenical council; he also presides over it or chooses someone else to do so and determines the agenda. The vacancy of the Holy See automatically suspends an ecumenical council. Laws or teachings issued by an ecumenical council require the confirmation of the pope, who alone has the right to promulgate them. The role of the pope in an ecumenical council is a distinct feature of the Catholic Church.
Plenary councils, which are meetings of the entire episcopate of a nation, are convoked by the national episcopal conference.
Provincial councils, which consist of the bishops of an ecclesiastical province smaller than a nation, are convoked by the metropolitan with consent of a majority of the suffragan bishops.

Plenary and provincial councils are categorized as particular councils. A particular council is composed of all the bishops of the territory as well as other ecclesiastical ordinaries who head particular churches in the territory. Each of these members has a vote on council legislation. Additionally, the following persons by law are part of particular councils but only participate in an advisory capacity: vicars general and episcopal, presidents of Catholic universities, deans of Catholic departments of theology and canon law, some major superiors elected by all the major superiors in the territory, some rectors of seminaries elected by the rectors of seminaries in the territory, and two members from each cathedral chapter, presbyterial council, or pastoral council in the territory. The convoking authority can also select other members of the faithful to participate in the council in an advisory capacity.
Meetings of the entire episcopate of a supra-national region have historically been called councils as well, such as the various Councils of Carthage in which all the bishops of North Africa were to attend.
During the Middle Ages, some councils were legatine, called by a papal legate rather than the pope or bishop.

Synods

Synods in Eastern Catholic Churches are similar to synods in Orthodox churches in that they are the primary vehicle for election of bishops and establishment of inter-diocesan ecclesiastical laws. The term synod in Latin Church canon law, however, refers to meetings of a representative, thematic, non-legislative or mixed nature or in some other way do not meet the qualifications of a "council". There are various types.
Diocesan synods are irregular meetings of the clergy and laity of a particular church summoned by the diocesan bishop to deliberate on legislative matters. Only the diocesan bishop holds legislative authority; the other members of the diocesan synod act only in an advisory capacity. Those who must be invited to a diocesan synod by law are any coadjutor or auxiliary bishops, the vicars general and episcopal, the officialis, the vicars forane plus an additional priest from each vicariate forane, the presbyterial council, canons of the cathedral chapter, the rector of the seminary, some of the superiors of religious houses in the diocese, and members of the laity chosen by the diocesan pastoral council, though the diocesan bishop can invite others to attend at his own initiative.

Episcopal conferences

National episcopal conferences are another development of the Second Vatican Council. They are permanent bodies consisting of all the Latin Church bishops of a nation and those equivalent to diocesan bishops in law. Bishops of other sui juris churches and papal nuncios are not members of episcopal conferences by law, though the conference itself may invite them in an advisory or voting capacity.
While councils and diocesan synods have full legislative powers in their areas of competence, national episcopal conferences may only issue supplementary legislation when authorized to do so in canon law or by decree of the Holy See. Additionally, any such supplemental legislation requires a two-thirds vote of the conference and review by the Holy See to have the force of law. Without such authorization and review, episcopal conferences are deliberative only and exercise no authority over their member bishops or dioceses.