What term refers to a meeting or formal assembly of delegates usually chosen for one session only?

The term that refers to a meeting or formal assembly of delegates usually chosen for one session only is "Convention." In the context of parliamentary procedure and formal assemblies, a convention typically gathers individuals who represent various groups or organizations to discuss specific topics, make decisions, or draft resolutions.

Conventions are often pre-planned events where delegates are appointed to represent their constituents, but the assembly is usually temporary, focusing on particular issues during that session. This distinguishes conventions from other meetings where participants might meet regularly or represent ongoing interests.

In contrast, a conference generally pertains to a broader gathering that may cover ongoing discussions and can involve multiple sessions. A symposium often involves academic or expert discussions on specific topics rather than a delegate assembly of varied representation. Lastly, a summit typically refers to a high-level meeting between prominent figures, often at the governmental or international level, which may not be focused on the delegate representation characteristic of conventions. Therefore, "Convention" accurately encapsulates the essence of a formal assembly with a set purpose and a temporary nature, making it the correct choice.