reflex

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

• adjective Physiology Being an involuntary action or response, such as a sneeze, blink, or hiccup.
• adjective Produced as an automatic response or reaction.
• adjective Bent, turned, or thrown back; reflected.
• adjective Reflexed.
• noun Physiology An involuntary response to a stimulus.
• noun A person's ability to respond to new or changing stimuli.
• noun Psychology An unlearned or instinctive response to a stimulus.
• noun Linguistics A form or feature that reflects or represents an earlier, often reconstructed, form or feature having undergone phonetic or other change.
• noun Something, such as light or heat, that is reflected.
• noun An image produced by reflection.
• noun A copy or reproduction.

from The Century Dictionary.

• To bend back; turn back.
• To reflect; cast or throw, as light; let shine.
• Thrown or turned backward; having a backward direction; reflective; reactive.
• In painting, illuminated by light reflected from another part of the same picture. See reflected light, under reflected.
• In oiol., bent back; reflexed.
• A sensation supposed to be produced by the irritation of an efferent or motor nerve: but the existence of the phenomenon is denied.
• noun Reflection; an image produced by reflection.
• noun A mere copy; an adapted form: as, a Middle Latin reflex of an Old French word.
• noun Light reflected from an illuminated surface to one in shade; hence, in painting, the illumination of one body or a part of it by light reflected from another body represented in the same piece. See reflected light, under reflected.
• noun Same as reflex action (which see, under reflex, a.).

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

• adjective Directed back; attended by reflection; retroactive; introspective.
• adjective Produced in reaction, in resistance, or in return.
• adjective (Physiol.) Of, pertaining to, or produced by, stimulus or excitation without the necessary intervention of consciousness.
• adjective (Physiol.) any action performed involuntarily in consequence of an impulse or impression transmitted along afferent nerves to a nerve center, from which it is reflected to an efferent nerve, and so calls into action certain muscles, organs, or cells.
• adjective (Physiol.) an excito-motory nerve. See Exito-motory.
• transitive verb obsolete To reflect.
• transitive verb To bend back; to turn back.
• noun Reflection; the light reflected from an illuminated surface to one in shade.
• noun (Physiol.) An involuntary movement produced by reflex action.
• noun See Knee jerk, under Knee.

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

• noun An automatic response to a simple stimulus which does not require mental processing.
• noun linguistics A corresponding phoneme in a daughter language.
• adjective Bent, turned back or reflected.
• adjective Produced automatically by a stimulus.
• adjective geometry Having greater than 180 degrees but less than 360 degrees.
• verb to bend, turn back or reflect
• verb to respond to a stimulus

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

• noun an automatic instinctive unlearned reaction to a stimulus
• adjective without volition or conscious control