Dictionary.com (v 1.1) Random House Unabridged Dictionary Inc. 2006.

Date of origin: circa 1400 - 1450.    
Boldness or daring, esp. with confident or arrogant disregard for personal
safety,  conventional  thought,  or  restrictions.  Effrontery  or  insolence;
shameless  boldness;  "His questioner's audacity shocked the lecturer."  

Synonyms:  nerve,  spunk,  grit,  temerity,  foolhardiness,  impudence,  
impertinence, or brashness.
Antonyms: discretion, or prudence.

The American Heritage® Dictionary of the English Language, 4th Edition
Copyright © 2006 by Houghton Mifflin Company. All rights reserved

Fearless daring; intrepidity,  bold or insolent heedlessness of restraints,  as
of those imposed by prudence, propriety, or convention. An act or instance
of  intrepidity  or  insolent  heedlessness. "warned  the  students  than  any
audacities committed during the graduation ceremony would be punished."

Online Etymology Dictionary © 2001 by Douglas Harper  

Boldness, brave, but more often "bold" in a bad sense;  "to dare,  be bold."

WordNet® 3.0, © 2006 by Princeton University

Fearless  daring,  aggressive boldness  or unmitigated effrontery;  "he  had
the audacity to question my decision."

The Definitions: