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Advanced Configuration and Power Interface

The ACPI specification allows an operating system to communicate with, and to control the power distribution to peripherals, such as hard drives. It was originally intended for laptops/notebooks where power saving is vital.

Since Windows 98, ACPI has replaced the older APM (Advanced Power Management) specification (used in Windows 95). Microsoft stopped supporting the APM specification in their operating systems from Windows Vista onwards. 

ACPI allows the operating system to communicate with the computer's BIOS and instruct the BIOS to power down peripherals. For example, when your computer goes into hibernation mode, the operating system is using the ACPI specification to control the power to the internal components.

http://www.helpwithpcs.com/jargon/acpi.htm