In the United States, a status conferred upon a person by the Defense Investigation Agency which allows them access to documents and other materials that have been classified confidential or higher by the U.S. Government or its contractors. Also known simply as a clearance. These include, in increasing order of secrecy and difficulty to obtain:
  • Secret
  • Top Secret
  • TS/CI -Top Secret Compartmentalized Information (thanks ekim yar!)
  • Code Word

...typically, even the titles of Top Secret documents are classified as secret.

In my personal case, a security clearance is the damned thing which means I am not able to smoke up. Unfortunately for me, I consider this restriction to be fairly reasonable from a strictly logical standpoint - the government basically said to me, "hey, if you want access to stuff that the general public can't see without reading Aviation Week, you'll need to sign this paper stating that you won't break the law." Now, you can (and I do) argue that the laws concerning da ganja in this country are fscking ridiculous; however, that's another discussion.

Update 9/11/02Delta-Sys asks the perfectly reasonable question: "How do you get one(?)"

Answer: there are several ways. By far the most common is to be hired for a position in which you have a need for one; in that case, your employer will 'sponsor' you for a clearance. The investigation required for a clearance is charged back to the employer (which usually means it's charged back to the government project paying said employer anyway). While a SECRET clearance process isn't too elaborate, mostly consisting of a background records check and fingerprinting, the higher levels will bring down all manner of surveillance, interviews of one's associates, penis length measurement and the like.

Another common means of getting a clearance is to work directly for the government in a capacity which requires you to handle classified material. Military personnel, for example, are cleared to handle sensitive tasks and information (anyone who touches the hot stuff for example). You could get elected; Shrubya has a TOP SECRET clearance (albeit probably not a Need to Know for various compartmentalized information). Traditionally, ex-Presidents retain their clearances, on the reasoning that their input and advice on sensitive matters in future is much more valuable than the (small) risk they will divulge the information.

Most clearances are related to the job, not the person; as long as the person has a need, the clearance is kept active. Once the person is no longer in a capacity where the clearance is being renewed, it will expire; however, for several years after that (seven, according to NotFabio) re-activation is much less onerous than a full investigation. Having a clearance, or having had one (and thus being able to probably renew it quickly and cheaply) can be a big plus in the government and associated job sectors.