The term tonic, also referred to as tonic note or tonic pitch, is a reference to the note or tone, on which a system of notes or tones (called scale or tonic harmony) is founded. The tonic and scale together define the musical key used in a song.
All other notes or tones that are used in a musical key either follow, or precede the tonic. A scale begins and ends with the tonic. Multiple scales can be based on the same tonic.