flinch

from The American Heritage® Dictionary of the English Language, 4th Edition

• intransitive v. To start or wince involuntarily, as from surprise or pain.
• intransitive v. To recoil, as from something unpleasant or difficult; shrink.
• n. An act or instance of starting, wincing, or recoiling.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. A reflexive jerking away.
• v. To make a sudden, involuntary movement in response to a (usually negative) stimulus.
• v. To dodge (a question), to avoid an unpleasant task or duty

from the GNU version of the Collaborative International Dictionary of English

• intransitive v. To withdraw from any suffering or undertaking, from pain or danger; to fail in doing or perserving; to show signs of yielding or of suffering; to shrink; to wince.
• intransitive v. To let the foot slip from a ball, when attempting to give a tight croquet.
• n. The act of flinching.

from The Century Dictionary and Cyclopedia

• To give way to fear or to a sense of pain; shrink back from anything painful or dangerous; manifest a feeling or a fear of suffering or injury of any kind; draw back from any act or undertaking through dread of consequences; shrink; wince: as, the pain was severe, but he did not flinch.
• In croquet, to allow the foot to slip from the ball in the act of croqueting.
• Same as flense.

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

• v. draw back, as with fear or pain
• n. a reflex response to sudden pain