A tonic key is a musical term that refers to the key of a piece of music where the tonic note or chord is the main focus. This creates a sense of resolution and stability in the music. However, there are different synonyms that can replace the term tonic key such as home key, primary key, principal key, or fundamental key. Each term represents the same idea of the key of a piece of music that centers around the tonic note or chord. Thus, these synonyms are essential to understand and use when discussing music theory and composition.