What is the significance of the term "define key" in music theory and how does it impact the overall composition of a musical piece?

In Music Theory, the term "define key" refers to the central note or chord that a piece of music is based on. The key sets the overall mood and tonal center of the composition, influencing the choice of notes, chords, and harmonies used throughout the piece. Changing the key can significantly alter the emotional impact and overall sound of the music.