What is "vivadi"

In Hindustani classical music, vivadi ( meaning " dissonant") refers to pitches that are not part of the arohana or avrohana of a given raga. In general, vivadi are not to be played during an improvisation on a given raga. So, for instance, if a raga had the structure S R G m P D N S', S' N D P m G R S; in that case, r, g, M, d and n would all be vivadi. Although vivadi pitches are generally to be avoided during the playing of a given raga, they are occasionally added by skilled performers in order to introduce some color to an interpretation.