It might be vague because it's a word that applies to a set of actions, whereas other words have more constricted definitions. But here is a basic moral baseline: don't steal. Now this has other implications as well, as not to steal life, sexual preference, bodily integrity, the fruits of one's labor. The concept applies beyond one action. Morality covers a lot. I view it as the most important yet most misunderstood subject in human history.