What is Special Session?

Legal Definition
In a legislature, a special session (also extraordinary session) is a period when the body convenes outside of the normal legislative session. This most frequently occurs in order to complete unfinished tasks for the year (often delayed by conflict between political parties), such as outlining the government's budget for the next fiscal year, biennium, or other period. Special sessions may also be called during an economic downturn in order to cut the budget. In other cases, a special session may be convened to address special topics, or emergencies such as war or natural disaster.

Who calls a special session varies - by vote of the legislature during regular session, by the executive, or by the legislature's speaker or presiding officer. The United Nations has both special sessions and emergency special sessions.
-- Wikipedia
Legal Definition
In English law. , A meeting of two or more justices of the peace held for a special purpose, (such as the licensing of alehouses,) either as required by statute or when specially convoked, which can only be convened after notice to all the other magistrates of the division, to give them an opportunity of attending. Stone, J. Pr. 52, 55.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary