To act unethically means to behave in a way that violates moral principles or standards of right and wrong. When someone acts unethically, they know their behavior crosses a line but choose to do it anyway, often to benefit themselves at others' expense.

A scientist acts unethically if she falsifies research data to make her results look better. A store owner acts unethically if he charges customers more than the marked prices. A student acts unethically if she takes credit for work her lab partner did. These actions break trust and fairness, the foundations of ethical behavior.

The word relates to ethics, which are the moral rules and principles that guide how people should treat each other. Different situations have different ethical standards: doctors follow medical ethics, lawyers follow legal ethics, and journalists follow journalistic ethics. But some ethical principles are universal: honesty, fairness, and respect for others matter everywhere.

Acting unethically is more serious than just breaking a rule. You might break a rule accidentally or without thinking, but unethical behavior involves knowing you're doing something wrong. When people discover someone has acted unethically, especially repeatedly, they lose trust in that person. A reputation for ethical behavior takes years to build but can be destroyed by a single unethical choice.