insider

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

• noun An accepted member of a group.
• noun One who has special knowledge or access to confidential information.

from The Century Dictionary.

• noun One who is inside; one who is within the limits of some place, structure, society, organization (as a church), etc.
• noun Hence One who has some special advantage, as in a business enterprise.

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

• noun an officer of a corporation or others who have access to private information about the corporation's operations, especially information relating to profitability.

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

• noun A person who has special knowledge about the inner workings of a group, organization, or institution.
• noun A person who is within an enclosed space.

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

• noun an officer of a corporation or others who have access to private information about the corporation's operations