FUD is Fear, Uncertainty, and Doubt (often spread on social media or mass media). FUD can cause the price of a coin to drop, not based on fundamentals or charts, but based on bad news that spreads around social media. Many times the bad news isn’t substantiated or grounded in reality, and instead ends up being something silly like a popular talking head’s opinion that Bitcoin is a bubble. The fear, uncertainty, and doubt-inducing idea being spread around media can be referred to as FUD.