Before launching into the beauty and magic of the art of extension, I would like to explain the different ways dance teachers and students use the vocabulary of 'extension.' 1. Extension Ballet Term Definition: Extension is a classical ballet term describing a dancer's ability to raise and hold a leg extended in the air. E.g. The height of the extension leg in developpe devant, or Sylvie Guillems six o'clock a la second, or the height of the back leg in an arabesque. 2.