vadi-samvadi

In the Hindustani classical music system, the vadi-samvadi is the sonant-consonant pair of notes in a raga. That is, the vadi is considered the most important note; the samvadi, placed around a fourth or fifth away from the vadi, is the second most important note. Both are frequently used, and often held for a long duration. In the alap, the performer will often center phrases around the vadi or the samvadi, building up tension before dissipating back into the tonic.

The concept of vadi and samvadi can be problematic; it is not uncommon for the exact location of the vadi or samvadi for a given raga to be in dispute. For an example, see Todi. However, when the vadi-samvadi is in dispute, it merely means that there are multiple ways of performng the raga.

Sources:

http://chandrakantha.com/articles/indian_music/vadi_samvadi.html
The Raga Guide, ed. Joep Bor. Published by Nimbus Records with the Rotterdam Conservatory of Music