There is no simple definition of music which covers all cases. It is an art form, and opinions come into play. Music is whatever people think is music. A different approach is to list the qualities music must have, such as, sound which has rhythm, melody, pitch, timbre, etc.
These and other attempts, do not capture all aspects of music, or leave out examples which definitely are music. According to Thomas Clifton, music is "a certain reciprocal relation established between a person, his behavior, and a sounding object".[1]p10 Musical experience and the music, together, are called phenomena, and the activity of describing phenomena is called phenomenology.