Bell's palsy

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

• noun A unilateral facial muscle paralysis of sudden onset, resulting from trauma, compression, or infection of the facial nerve and characterized by muscle weakness and a distorted facial expression.

from the GNU version of the Collaborative International Dictionary of English.

• Paralysis of the facial nerve, producing distortion of one side of the face.

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

• noun pathology A paralysis of cranial nerve VII (the facial nerve) resulting in inability to control facial muscles on the affected side.