debated. Commonly identified aspects include pitch (which governs melody and harmony), duration (including rhythm and tempo), intensity (including dynamics) and timbre. Though considered a cultural universal, definitions of music vary wildly throughout the world as they are based on diverse views of nature, the supernatural, and humanity. Music is often differentiated into composition and performance, while musical improvisation may be regarded as an intermediary tradition. Music can be divided into genres and subgenres, although the dividing lines and relationships between music genres are often subtle, sometimes open to individual interpretation, and occasionally controversial. Theatre Theatre or theater (from Greek theatron (θέατρον); from theasthai, "behold") is the branch of the performing arts concerned with acting out stories in front of an audience using combinations of speech, gesture, music, dance, sound and spectacle – indeed, any one or more elements of the other performing arts. In addition to the standard narrative dialogue style, theatre takes such forms as opera, ballet, mime, kabuki, classical Indian dance, Chinese opera and mummers' plays. Multidisciplinary artistic works Areas exist in which artistic works incorporate multiple artistic fields, such as film, opera and performance art. While opera is often categorized in the performing arts of music, the word itself is Italian for "works", because opera combines several artistic disciplines in a singular artistic experience. In a typical traditional opera, the entire work uses the following: the sets (visual arts), costumes (fashion), acting (dramatic performing arts), the libretto, or the words/story (literature), and singers and an orchestra (music). The composer Richard Wagner recognized the fusion of so many disciplines into a single work of opera, exemplified by his cycle Der Ring des Nibelungen ("The Ring of the Nibelung"). He did not use the term opera for his works, but instead Gesamtkunstwerk ("synthesis of the arts"), sometimes referred to as "Music Drama" in English, emphasizing the literary and theatrical components which were as important as the music. Classical ballet is another form which emerged in the 17th century in which orchestral music is combined with dance. Other works in the late 19th, 20th and 21st centuries have fused other disciplines in unique and creative ways, such as performance art. Performance art is a performance over time which combines any number of instruments, objects, and art within a predefined or less well-defined structure, some of which can be improvised. Performance art may be scripted, unscripted, random or carefully organized; even