Taking immediate advantage, often unethically, of any circumstance of possible benefit.
Designating or of an infection or disease caused by a microorganism (opportunistic microorganism) that is normally not virulent but that can cause a serious condition in those with a weakened immune system.
Webster's New World
The definition of opportunistic is an action or judgment that furthers a person's own interests.
An example of opportunistic is a woman telling a friend a false rumor about her boyfriend cheating on her, to try to end their relationship.
Said of people who will take advantage of situations to advance their own interests, without regard for principles.