noun

• (music, informal) An arpeggiator.
• (music, informal) An arpeggio.
• A surname.
• (physiology) Initialism of absolute refractory period.
• (Britain, dated) Short for Air Raid Precautions.
• (networking, Internet) Initialism of Address Resolution Protocol. [(networking, Internet) The protocol used to translate IP addresses into MAC addresses to support communication on a LAN.]