Definition and background:
The term used to describe the musical style of the late 14th century, specifically that of French composers such as Johannes Cuvelier, Philippus de Caserta, and Jacob de Senleches, who lived after Guillaume de Machaut. These composers refined the notational features of the ars nova period to produce a more sophisticated and more rhythmically complex style. ("Subtle art") a modern term referring to music from late fourteenth- and early fifteenth-century France. Characteristics include intricate rhythms, exotic harmonies, and erudite poetry.
For more information on the music term "Ars subtilior" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary