Arpeggiation is the process of outlining a vertical sonority as a melodic figure. This may be in ascending or descending order or may leap back and forth. Arpeggiation of any interval beyond the third will often create a ‘polyphonic line’, i.e. a single line that suggests several voice-leading patterns.
Unless the arpeggiation is very long and virtuosic, make sure your arpeggiation and polyphonic melody expresses an actual voice-leading pattern. This is especially important in cadences – where the leading tone should resolve. However, it is also important in sequences and other progressions.