Opportunism is taking advantage of situations and people in a self-centered way, without regard to the consequences.
An opportunity is a chance for something beneficial, like a job or a new romance. Opportunities are good things, but someone who pursues opportunities a little too eagerly is guilty of opportunism. Opportunism is a self-serving, self-centered approach to situations that disregards how others feel. Opportunism doesn't necessarily break the law, but it does violate manners and decency. Moving in on a dead person's wife is a classic example of opportunism. People who act this way are called opportunists.