The first note of a musical Scale, also called the fundamental.
A scale is composed of 8 notes (7 and the Octave):
The Tonic is important as the starting and ending note of a piece of music, and in forming important Chords along with the Dominant and the Mediant. Although not always adhered to, the basic principal of Melody has a tune begin on the Tonic, wander where it will and return (resolve) to the Tonic at the end for a feeling of completeness. The Leading Note therefore always produces a sense of wanting to be resolved. Each note gives a sense of movement or stability to the passage and some, like the Leading Note want to move on to a more stable note while others like the Dominant are relatively stable already.