The term 'LACKING A KEY, IN MUSIC' can be summarized as 'ATONAL, ' which is a type of music that does not conform to any key or tonality. ATONAL music is composed without any reference to a key or tonal center, and often uses chromaticism and dissonance to create an unsettling sound. It was developed in the early 20th century by composers such as Arnold Schoenberg and Anton Webern, and has been used in various genres of music since then.