A controversy is a dispute, argument, discussion or debate featuring strong disagreements and opposing, contrary, or sharply contrasting opinions about an idea, subject, group or person. A controversy can range in scope from private disputes between two individuals, such as claims to property, to large-scale disagreements between societies.

The term originates circa 1384 from Latin controversia, as a composite of controversus - "turned in an opposite direction," from contra - "against" - and vertere - to turn, or versus (see verse), hence, "to turn against."

Benford's law of controversy, as expressed by science-fiction author Gregory Benford in 1980, states: "Passion is inversely proportional to the amount of real (true) information available." In other words, the more untruths the more controversy there is, and the more truths the less controversy there is.

A controversy is always the result of either ignorance (lack of sufficient true information), misinformation, misunderstandings, half-truths, distortions, bias or prejudice, deliberate lies or fabrications (disinformation), opposed underlying motives or purposes (sometimes masked or hidden), or a combination of these factors.

There is often controversy about a significantly different new idea, subject, group or person for a period of time, until honest direct examination of sufficient factual evidence results in widespread agreement on the truths of the matter. Examples: disease is often caused by bacteria or viruses, rather than evil spirits or bad blood; blood is circulated around the body, rather than ebbing and flowing; Earth revolves around the sun, rather than being the center of the universe; Earth is a sphere, rather than flat.

Religion today is considered controversial by many atheists and non-theists around the world.

Neither “reliable sources” nor “multiple reports” are dependable ways to resolve any controversy, because for centuries, opponents in military and industrial espionage, politics, religion, public relations, and national economies, have used multiple reports and reliable sources of information to manipulate people in war, business, elections, beliefs, public opinion and financial markets – using disinformation or Black propaganda planted secretly or underhandedly, or passed on unwittingly, through double-agents, con men, radio broadcasts, news media, authoritative writings and other means, including gossip and the internet in modern times.

Courts of law have long recognized this fact. (See further discussion regarding law below.) Thus, decent and fair courts don't allow 2nd-hand hearsay evidence (“I heard John say that he heard Mary say so and so”), and require opinion to be clearly labeled as opinion not fact, and instruct juries to base their findings more on hard evidence (like DNA tests and crime scene re-enactments) than on testimony alone, since multiple witnesses can give the same flawed ideas or perceptions, or the same intentional lies and distortions.