Nicomachean Ethics is the name of Aristotle's best known work on ethics. It is named after his son, Nicomachus, because it is believed to have been recorded by him. Scholars believe that Nicomachus compiled writings of his father's that were not actually meant to be published.
The book, of course, is too long to summarize easily in this space. Overall, we can say that the book argues that the greatest human good is happiness and that all people should work towards achieving happiness. However, it is important to note that this does not mean that we should just do whatever we want. Instead, we are supposed to act in the right ways and for the right reasons. Doing so is what will make us happy.
It is in this book that Aristotle famously argues that all things are good when done to the right extent. All things can be bad when done too much or too little, but are good when done to the right extent.
So, a very brief definition of Nicomachean Ethics is that it is Aristotle's major ethical work, in which he argues that acting in moderation, doing the right things for the right reasons, will make us happy. This is what we should strive for in our lives.