The main set of pitches of a piece of music
"Key" refers to the main tonality of a piece of music, and it serves as the foundation for both the melody and harmony. It can be broadly divided into two categories - major key and minor key. Both create different effects on the atmosphere and emotions conveyed.
Generally, major key is usually associated with liveliness and cheerfulness, and often used to express positive emotions. On the other hand, minor key is usually associated with sadness and darkness, and often used to convey sorrowful emotions.