Definition: In Congress, the seniority system describes the practice of granting privileges to members of Congress (Senators and Representatives) who have served the longest. Those privileges range from choice of offices to choice of committee assignments, for example. In addition, members with a longer term of service on a committee are assumed to be "senior" and have more power in the committee. Seniority is usually, but not always, considered when each party awards committee chairmanships, the most powerful position on a committee.
Because of the nature of the terms of office, seniority is more important in the Senate (six year terms) than in the House of Representatives (two year terms). In 1995, Senate Republicans voted to limit committee chairmanships to a six years.
However, the most public leadership positions -- Speaker and Majority Leader in the House and Majority and Minority Leader in the Senate -- are elected positions.