"We can define a conflict of interest as a situation in which a person has a private or personal interest sufficient to appear to influence the objective exercise of his or her official duties as, say, a public official, an employee, or a professional."

Source: Chris MacDonald, Michael McDonald, and Wayne Norman, “Charitable Conflicts of Interest”, Journal of Business Ethics 39:1-2, 67-74, August 2002. (p.68)

For advice on devising a conflict of interest policy for your organization, see: "Writing a Conflict of Interest Policy"