comic relief

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A humorous or farcical interlude in a serious literary work or drama, especially a tragedy, intended to relieve the dramatic tension or heighten the emotional impact by means of contrast.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun narratology The inclusion of a humorous character or scene or witty dialogue in an otherwise serious work, often to relieve tension.