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.
PulseWidthModulation, referring to an oscillation or modulation of the width of the pulse waveform, causing an audible movement to the sound produced. This form of synthesis was present on various early synthesizers, and is still used today.
The Moog modular synthesizer had a PWM module attached to the VCO.