Music is a form of art that involves organized and audible sounds and silence. It is normally expressed in terms of pitch (which includes melody and harmony), rhythm (which includes tempo and meter), and the quality of sound (which includes timbre, articulation, dynamics, and texture). Music may also involve complex generative forms in time through the construction of patterns and combinations of natural stimuli, principally sound. Music may be used for artistic or aesthetic, communicative, entertainment, or ceremonial purposes. The definition of what constitutes music varies according to culture and social context.