Stands for "Power On Self Test." POST (or P.O.S.T.) is a series of system checks run by computers and other electronic devices when they are turned on. The results of the test may be displayed on a screen, output through flashing LEDs, or simply recorded internally. On computer systems, the POST operation runs at the beginning of the boot sequence. If all the tests pass, the rest of the startup process continues automatically.

Both Macs and Windows PCs run a POST each time the computer is booted up or restarted. The scan checks the hardware and makes sure the processor, RAM, and storage devices are all functioning correctly. If an error is encountered during the POST, the startup process may pause or halt completely and the error may be displayed on the monitor. On PCs, POST errors are often displayed on the BIOS information screen. They may be output as cryptic codes, such as "08" for bad memory, or as a system message, such as "System RAM failed at offset." On Macs, POST errors are often indicated by a simple graphic, such as a broken folder icon that indicates no bootable device was found.

In some cases, the computer screen may not even turn on before POST errors take place. If this happens, error codes may be output through flashing LED lights or audible tones. For example, an Apple iMac will sound three successive tones, followed by a five second pause, then repeat the tones when bad RAM is encountered during startup. Most PCs also emit beeps when POST errors are detected, though each manufacturer uses its own codes.

POST is rather technical term that only computer technicians use on a regular basis. However, it is a good acronym to know, since it will help you better understand error messages that may pop up on computers or other electronic devices. If your computer won't start up because of a POST error, you can use a different device to look up the meaning and cause of the error, possibly from the manufacturer's website. Then you can take the appropriate action, such as removing a memory module or reseating the video card, then you can try starting up your computer again.

NOTE: "POST" is also a method for passing HTML form variables from one webpage to another without displaying them in the address bar. The alternative method is "GET," which appends the values to the URL.

Updated November 24, 2012 by Per C.

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