Insider trading can simply be defined as taking advantage of trading information that was not made public. This is especially when the person is an insider or employee.
Definition: Insider trading is defined as a malpractice wherein trade of a company's securities is undertaken by people who by virtue of their work have access to the otherwise non public information which can be crucial for making investment decisions.
Insider trading is an unfair practice, wherein the other stock holders are at a great disadvantage due to lack of important insider non-public information. However, in certain cases if the information has been made public, in a way that all concerned investors have access to it, that will not be a case of illegal insider trading.