An opportunist is someone who takes advantage of situations to benefit themselves, often without worrying much about principles or how their actions affect others. When a student becomes friends with the smartest kid in class right before a big project is due, then ignores them afterward, that's opportunistic behavior.

The word often carries a negative tone because opportunists seem more interested in what they can get than in being consistent or fair. A politician might be called an opportunist if they change their positions based on whatever will win votes, rather than standing by their beliefs. In business, an opportunist might jump from one trend to another, always chasing the next quick profit.

However, the word isn't always negative. Sometimes it simply means someone who recognizes and seizes good chances when they appear. A quarterback who spots an unexpected opening and runs for a touchdown is being opportunistic in a positive way. Scientists can be opportunists when they notice an unexpected result in an experiment and pursue it, leading to important discoveries.

The key difference lies in whether someone is genuinely making the most of circumstances or using situations purely for selfish gain. The first shows alertness and initiative. The second shows a willingness to use people or abandon principles whenever it's convenient.