The annual series of meetings of a Congress is called a session. Each Congress generally has two sessions, based on the constitutional mandate that Congress assemble at least once a year. In addition, a meeting of one or both houses is a session. And a house is said to be in session on any particular day when it is meeting.
Closed sessions of the Senate, sometimes referred to as secret sessions, are used for deliberations during impeachment trials, as well as to discuss issues of national security, confidential information, and sensitive communications from the president. Prior to 1795, every session of the Senate was closed.
A lame duck session occurs when Congress (or either chamber) reconvenes following the November general elections. Some lawmakers who return for this session will not be in the next Congress because they were defeated or decided not to run again. Hence, they are informally called "lame duck" members participating in a "lame duck" session. When the president convenes a session after Congress has adjourned sine die, it is called a special session.