In Islam, term denoting both the political and miltiary system controlled by the Caliph, as well as the symbolic institution of the Caliph. In effect this involves a state-like ruling structure for a Muslim state, headed by a single male ruler with powers and authorities defined to be invested in him by God.
The caliphate begins in 632 CE and lasts effectively until 1258, but is recreated by the Ottoman rulers in the 16th century, and lasts as a formal institution until 1924.
The Caliphate came into its ideal shape through the 7th century, becoming a centralized institution with structures very close to that of a royal throne. During its first 2 centuries, the caliphate was an acting political power, but groups around the caliph would over time see that their direct influence on active politics became stronger to the point that the caliph was little more than a symbolic figure. As it happened, the caliph would actually become less of a political figure than most contemporary kings.
As the caliphate was centered around the Caliph, it also made claim to be a ruling institution founded by the will of God. The caliphate was considered a continuation of the governance of Muhammad, who Muslims consider to be the prophet of Islam.
The Caliphate claim on being an institution of God, is far from uncommon. Most kings of earlier and later times made similar claims, there could be a claim that the king was a god, that he was the son of a god, or that he was appointed and blessed by a god.
In modern Islamism
In modern times, the concept of "caliphate" has become a central motive in Islamist rhetorics. Based upon failing understanding of actual history, many radical Muslims have come to define the reestablishment of the caliphate was the ultimate goal of Muslim strivings. This goal bases itself upon a factual understanding unfounded by Muslim sources, and non-Muslim sources alike, that a single ruler fully training in Islamic sciences will be effectively Muslim world, in which there is peace and no injustice. By this, Islamism claims to recreate a political situation that never has existed.
As to the questions who should become the actual caliph, what powers he should exercise, there comes no clear answer from the many Islamist groups. This is due to the fact that the central idea of Islamism's politics is that all shall be ruled according to God, not man. If a true leadership is installed, the world will be governed according to God's will, and from comes that human decisionmaking is unnecessary. One aspect of the future Caliphate is that it will not be democratic, yet it may have a council structure, a structure were voices of the people will address the ruler asking for his aid.
Location of the caliphate
The actual location of the caliphate could at times be unclear, as some periods of Islamic history had the caliph heading military campaigns, campaigns that could last years. Five cities across the Muslim world have at periods housed caliphate courts, hence been symbolically the capitals of Islam. Mecca, which was the central city in the early decades, and always the symbolic centre of Islam, never had a formal caliphate court, and cannot therefore be listed among the other ones. The cities were Damascus; Baghdad; Samarra; Cairo; and Istanbul (long known as Constantinople).
632-661: Changing, but roughly with Mecca as the main headquarters.
661-750: Damascus; this was a period of permanent location of the caliphate and its office.
750-762: No specific location; the new Abbasid caliphs roamed the Mesopotamian lands.
762-836: Baghdad, again permanent, this time with a lavish court established. This was also a period in which new political structures emerged, thereby replacing the active power of the caliph.
836-892: Samarra becomes the new headquarters of the caliphate.
892-1258: Relocated back to Baghdad.
1258-1261 No caliph.
1261-1517: Cairo. The caliphate at this point is bereft of any political importance.
1517-1924: Constantinople; the city that gradually would change its name to Istanbul. The title "caliph" was now taken by the Ottoman sultan, recreating some legitimacy for the institution, but this was objected to by many Muslims.