Fear is an emotion sensed by most humanoids. It is initiated when an individual perceives a danger, such as a threat of injury or death. The fear of a specific object, situation, person, or group is known as a phobia.
Within Humans, fear leads to the release of adrenaline levels within the body. Pavel Chekov was spared the effects of radiation-induced hyperaccelerated aging when the release of adrenaline from his momentary fear of discovering a dead body on Gamma Hydra IV protected him from the radiation on the planet. (TOS: "The Deadly Years")
In conversation with Lt. Commander Data, Captain Jean-Luc Picard remarked that "fear can be a powerful motivator." In Charles Dickens's story, "A Christmas Carol," fear is used to motivate the protagonist, Ebeneezer Scrooge to reform his character, by showing him that an eternity of torment awaits him if he continues with his current character. By the same token, he speculates, the early ancestors of the people of Ventax II used the legend of Ardra, a mythical figure similar to Earth's Devil, to inspire fear in support of planetary wide reforms. Unfortunately, a con artist posing as Ardra was able to exploit this same fear a thousand years later. (TNG: "Devil's Due")
Franklin D. Roosevelt once said, "The only thing we have to fear is fear itself." Harry Kim discovered that this is literally true when he encountered the psychopathic Clown, an embodiment of the emotion. (VOY: "The Thaw")