The______ is an essential characteristic of the sonata form.

Answer 1

http://en.wikipedia.org/wiki/Sonata#The_sonata_idea_or_principle

http://en.wikipedia.org/wiki/Sonata_form

Read carefully.

As with all Wikipedia search results, be sure to check out all the external links and further references at the bottom of each Wikipedia page.

Answer 2

There are about a hundred terms that can fit here. I am not certain any of them are "essential", but some would argue that. If I had to pick one term, it would be "musical theme". I doubt if that is what your teacher wants.