shudder

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

• intransitive verb To shiver convulsively, as from fear or revulsion. synonym: shake.
• intransitive verb To vibrate; quiver.
• noun A convulsive shiver, as from fear or revulsion.
• noun A vibration or trembling motion.

from The Century Dictionary.

• To shake; quiver; vibrate.
• Hence, in particular, to tremble with a sudden convulsive movement, as from horror, fright, aversion, cold, etc.; shiver; quake.
• To have a tremulous or quivering appearance, as if from horror.
• Synonyms Quake, etc. See shiver.
• noun A tremulous motion; a quiver; a vibration.
• noun Specifically, a quick involuntary tremor or quiver of the body, as from fear, disgust, horror, or cold; a convulsive shiver.

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

• intransitive verb To tremble or shake with fear, horrer, or aversion; to shiver with cold; to quake.
• noun The act of shuddering, as with fear.

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

• noun A shivering tremor.
• noun A moment of almost pleasurable fear; a frisson.
• verb intransitive To shake nervously, as if from fear.
• verb intransitive To vibrate jerkily.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an almost pleasurable sensation of fright
• verb tremble convulsively, as from fear or excitement
• verb shake, as from cold
• noun an involuntary vibration (as if from illness or fear)