A joint session or joint convention is, most broadly, when two normally-separate decision-making groups meet together, often in a special session or other extraordinary meeting, for a specific purpose.
Most often it refers to when both houses of a bicameral legislature sit together. A joint session typically occurs to receive foreign or domestic diplomats or leaders, or to allow both houses to consider bills together.
The State of the Union Address of the president of the United States is traditionally made before a "joint session" of the United States Congress. Many states refer to an analogous event as a "joint convention". Such assemblies are typically held in the chamber of the lower house as the larger body.
State constitutions of U.S. states may require joint conventions for other purposes; for example Tennessee's requires such to elect the secretary of state, the state treasurer, and the comptroller of the treasury.
A similar event is the speech from the throne upon the state opening of Parliament in Commonwealth Realms is made before a joint sitting of the both Houses. In most cases this event occurs in the Upper Chamber, due to the British constitutional convention that the monarch never enters the Commons chamber. An exception was the Governor-General's Address in the Irish Free State, which was made to the lower house, with Senators invited to attend.
The Canadian government procedure is called a joint address.
Various government agencies and non-governmental organizations may also meet jointly to handle problems which each of the involved parties has a stake in.
Some Constitutions give special power to a joint session, voting by majority of all Members of the Legislature regardless of which House/ chamber they belong to. For example, in Switzerland a joint session of the two houses elects the members of the Federal Council (cabinet). In India, disputes between Houses are resolved by a joint sitting but without an intervening election.
In Australia, if a Bill is deadlocked, then following a double dissolution of both Houses, and if the new Parliament is still unable to pass the Bill, then the Bill may be passed by an absolute majority of a joint sitting. The only such example of this occurring was the Joint Sitting of the Australian Parliament of 1974 under the Whitlam Labor government.
The Congress of France is an assembly of both houses of the French Parliament, convened at the Palace of Versailles, which can approve certain amendments to the constitution by a three-fifth majority of all members. Since 2008, the Congress may also be convened to hear an address from the President of the Republic.