When an unscheduled session of congress is called by the president, it is called a(n)

Special session, is the right answer.

Explanation:

In the context of a legislature, a special session is a period during which the body assembles outside of the ordinary parliamentary sitting. This special session occurs to achieve uncompleted assignments for the year, for instance, drafting the budget of the government for the next year. This session may also occur at the time of economic crisis to cut the budget and to address some critical issues such as natural disaster and war etc.

assessment is the missing word