List of U.S. security clearance terms

Description.
Security clearance levels are used as part of a method to control access to information that should not be freely available to all personnel.
The different organizations in the United States Federal Government use different terminology and lettering, as is discussed below.
Use.
Security clearance levels often appear in employment postings for Defense related jobs, and other jobs involving substantial amounts of responsibility, such as air traffic control or nuclear energy positions. Employers generally prefer to hire people who are already cleared to access classified information at the level needed for a given job or contract, because security clearances can take up to a year to obtain. In general, most employers look for candidates who hold an active Department of Defense (DoD) collateral clearance or a blanket TS/SCI-cleared (Top Secret / Sensitive Compartmented Information) and who have a counterintelligence (CI), full-scope polygraph (FSP), also known as expanded scope screening (ESS).
Security levels.
Security clearances can be issued by many United States of America government agencies, including the Department of Defense (DoD), the Department of State (DOS), the Department of Homeland Security (DHS), the Department of Energy (DoE), the Department of Justice (DoJ), the National Security Agency (NSA), and the Central Intelligence Agency (CIA). DoD issues more than 80% of all clearances. There are three levels of DoD security clearances:
Additionally, the United States Department of Energy issues two levels of security clearances:
Despite the common misconception, a public trust position is not a security clearance, and is not the same as the confidential designation. Certain positions which require access to sensitive information, but not information which is classified, must obtain this designation through a background check. Public Trust Positions can either be moderate-risk or high-risk.
Information "above Top Secret," a phrase used by the media, means either Sensitive Compartmented Information (SCI) or Special Access Program (SAP). It is not truly "above" Top Secret, since there is no clearance higher than Top Secret. SCI information may be either Secret or Top Secret, but in either case it has additional controls on dissemination beyond those associated with the classification level alone. In order to gain SCI Access, one would need to have a Single Scope Background Investigation (SSBI). Compartments of information are identified by code words. This is one means by which the "need to know" principle is formally and automatically enforced. 
In order to have access to material in a particular SCI "compartment", the person must first have the clearance level for the material. The SCI designation is an add-on, not a special clearance level. Someone cleared at the SECRET level for some compartment X cannot see material in compartment X that is classified TOP SECRET. But the reverse is not true: a person cleared for TOP SECRET with access to X material can also access SECRET material in compartment X. The codeword flags for SECRET and TOP SECRET material in every compartment are different, and each codeword is classified at the level it protects (the SECRET codeword for a compartment is itself classified SECRET, etc.) 
As long as the holder of a clearance is sponsored, the clearance remains active. If the holder loses sponsorship, the holder is eligible for re-employment with the same clearance for up to 24 months without reinvestigation, after which an update investigation is required.
A Periodic Reinvestigation is typically required every five years for Top Secret and ten years for Secret/Confidential, depending upon the agency. Access to a compartment of information lasts only as long as the person's need to have access to a given category of information.
Unclassified (U) is a valid security description, especially when indicating unclassified information within a document classified at a higher level. For example, the title of a Secret report is often unclassified, and must be marked as such. Material that is classified as Unclassified // For Official Use Only (U//FOUO) is considered between Unclassified and Confidential and may deal with employee data.
For access to information at a given classification level, individuals must have been granted access by the sponsoring government organization at that or a higher classification level, and have a need to know the information. The government also supports access to SCI and SAPs in which access is determined by need-to-know. These accesses require increased investigative requirements before access is granted.
Investigations.
The following investigations are used in clearance determinations:
Many other investigative products have been used to grant clearances in the past. While some of them are still used to determine suitability for employment or enlistment, only the above are used to grant clearances.
Additional investigation or adjudication.
Certain accesses require persons to undertake one or more polygraph tests:
SCI eligibility.
Sensitive compartmented information (SCI) is a type of classified information controlled through formal systems established by the Director of National Intelligence. To access SCI, one must first have a favorable SSBI and be granted SCI eligibility. Because the SSBI is also used to grant collateral top secret eligibility, two are often granted together and written TS/SCI. Access to individual SCI control systems, compartments, and subcompartments may then be granted by the owner of that information. Note that additional investigation or adjudication may be required.
In general, military personnel and civilian employees (government and contractor) do not publish the individual compartments for which they are cleared. While this information is not classified, specific compartment listings may reveal sensitive information when correlated with an individual's résumé. Therefore, it is sufficient to declare that a candidate possesses a TS/SCI clearance with a polygraph.