An Acronym for Advanced Configuration and Power Interface. The newer method for changing devices' power status (eg on standby, hibernating, etc). The older method was APM (Advanced Power Management).
Some example uses:
Many newer laptops use ACPI exclusively without support for APM. Linux kernel 2.4.x had some support for ACPI, but kernel 2.6 should have much better support.
3 pages link to ACPI: