Ethics is the study of morality in an objective or socially universal context. It looks at questions such as whether or not a given moral system is harmful or beneficial, whether or not given moral value are helpful, and so forth.
Put another way, ethics is meta-morality.
It might be described as "objective morality", where the ultimate moral value is some function of the overall benefit or harm to society of a given rule or action.
While there is no perfectly objective morality, ethical analysis must always answer to that criterion as its ultimate value, where other moral systems may state rules as absolutes or cite scripture or other authority -- which in a rational context would be considered argument from authority. Given this, "ethics" might also be referred to as non-authoritarian (or anti-authoritarian) morality.
This is arguably a separate usage of the word; following from the definition of "ethics", "unethical" should mean "not subjected to ethical analysis" or "outside the realm of ethical consideration" rather than "determined to be harmful".