An acronym for AR Pistol, a variant of the AR-15 rifle or carbine. AR pistols are defined as any caliber firearm in the AR platform with a barrel shorter than 16 inches. ARPs do not have a butt stock and are sold with only a buffer tube at the end, often covered with foam.
An Arp, short for Arpeggiator, is a device that causes a synthesizer to play the all of the notes in a chord in an arpeggio, rather than in harmonies. It was frequently present on monophonic synthesizers, and was used in early Italo Disco to create that "galloping synth" effect.
The modular synthesizer had an arp module, allowing it to play chord patterns which were impossible beforehand.