Seniority is the concept of a person or group of people taking precedence over another person or group because the former is either older than the latter or has occupied a particular position longer than the latter. Seniority is present between parents and children and may be present in other common relationships, such as among siblings of different ages or between workers and their managers.
Under a seniority system, control is often granted to senior persons due to length of service in a given position. When persons of senior rank have less length of service than their subordinates, "seniority" may apply to either concept.
In some military command structures, the length of time someone has held a particular rank is called "seniority in grade" and determines whether that person is senior to another person of the same rank. For instance, a captain who was promoted five years ago can give orders to a captain who was promoted three years ago.
Seniority in United States politics, when used out of context, is informally defined as the number of years one member of a group has been a part of the group. For example, as of January 2015, John Conyers from Michigan is the most senior member of the House of Representatives. However, "seniority" can also refer to political power attained by position within the United States Government. For further details, see:
Seniority is viewed sometimes both positively and negatively. Many elected officials are viewed as retaining their position only because they have been there for many years, which can reflect voter stagnancy and the benefits of incumbency. On the other hand, long years of incumbency can also be seen as a sign of the person's ability to continue pleasing voters or the use of seniority to deliver benefits to constituents.
In some countries the Dean of the Diplomatic Corps receives special treatment.
Seniority also has an influence over bumping rights, which is a reassignment of jobs, possibly for many people at a time.
Some traditionalist employers, common in smaller, single-operated business, take a "last in, first out" (LIFO) perspective, meaning those who have been there longest or who have tenure have the right to stay, whereas other employers take a "first in, first out" (FIFO) or "inverse seniority" viewpoint, which tends to emphasize a new or "fresh start" for the company.
Seniority does have several positive factors to its name. Individuals may be drawn toward a specific field or occupation with the knowledge that seniority is obtainable. If seniority were to be banished as a whole, many higher paid employees would be fired first just because they make more money than their peers. Seniority does an effective job in helping people, interested in staying at one organization, in working towards having a "marathon" career. One of the goals of a seniority system is employee retention, which ensures an organization is retaining institutional knowledge, erudite employees, and an opportunity for mentorship of new hires. It's important to make sure employees are here to stay.
Though the principle of seniority does an effective job of protecting long-term employees, in some scenarios, it can fail to address several critical factors. Firstly, spots secured by seniority casts aside some of the most appealing perspectives. Individuals will become less driven to enter a field that does not reinforce their efforts with employment. Secondly, the security of tenure often encourages mediocrity.  Employees with the knowledge that their spot in the workplace is secured will naturally become less likely to improve their working ethics as they may no longer view improvement as a necessity. Lastly, a system rewarding individuals for their hiring date does not encourage professional growth. If individuals are aware they only have to reach a certain time-span of employment to have a guaranteed position in a company, they will not grow professionally once they have reached their mark. 
During the late 20th century in the United States, the federal government stopped emphasizing seniority as the primary basis for pay raises for its workforce. The Reagan administration replaced a seniority-based system for pay increases for its white collar government workers. The new system included performance appraisal. 
Commercial aviation pilots working for a carrier have their privileges determined by their seniority or generally known as the "pilot seniority list." These privileges can be income level, routes flown, types of aircraft, work schedules and positions. Seniority is most important when deciding which pilots to upgrade to a larger, more complex aircraft type; or for upgrading a First Officer to the rank of a Captain.
Engine drivers with many railways also have a seniority list, but it is focused on work scheduling. Younger engine drivers often serve as back-up personnel and must help out on a very short notice - for example when a colleague calls in sick or has a delay.