An arpeggio is actually the
technique of playing the notes from a
chord in rapid succession. Not so fast that you hear every note of the chord at once, that would be
strumming, but slowly and sharply, making each note clear.
Arpeggios don't have anything to do with scales as mentioned in an earlier node, and I'm not sure what an "outline of a
scale" is, exactly.
In regards to
guitar:
The arpeggio technique can be applied to more than one chord at a time using a technique called
sweep picking, where you change the
fingering of the chord you are making halfway through the arpeggio so as to retain the arpeggiated
sound and make a chord that would be
impossible to make normally, perhaps because you'd have run out of fingers when trying to make it while strumming, or because you'd be
barring notes that you don't want to play.