Machiavelli's "The Prince" will give you a decent understanding of what people usually mean by "Machiavellian". The book explains what methods would allow an absolute ruler to stay in control of state. It does not generally make moral judgments about those methods.
Machiavelli's "Discourses" is the one that will really confuse a reader looking to understand the colloquial meaning of "Machiavellian". In this book, Machiavelli lays out a vision of a healthy "republic" (or more precisely, res publica) which benefits the people who live in it. Among other things, Machiavelli argues that republics actually benefit from multiple competing factions, and from some kind of checks and balances. Apparently these ideas influenced several of the people who helped draft the Constitution of the United States.
Now why Machiavelli had two such different books on how governments worked is another interesting question...
Ymmv, but I think that's far from the point of the book, and isn't even the main topic. It's hard for me to imagine taking a person who'd never heard the term, letting them read the book, and then asking them to propose a definition, would produce anything like the above.
Machiavelli's "Discourses" is the one that will really confuse a reader looking to understand the colloquial meaning of "Machiavellian". In this book, Machiavelli lays out a vision of a healthy "republic" (or more precisely, res publica) which benefits the people who live in it. Among other things, Machiavelli argues that republics actually benefit from multiple competing factions, and from some kind of checks and balances. Apparently these ideas influenced several of the people who helped draft the Constitution of the United States.
Now why Machiavelli had two such different books on how governments worked is another interesting question...