Fear is an emotion sensed by most lifeforms. 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, Ebenezer 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. Captain Kathryn Janeway, meanwhile, sought a way to reach the Clown, wondering whether there was more to fear than just a demand to simply exist. The Doctor noted that fear could provide pleasure by allowing one to seek the boundaries of one's existence and Janeway pondered "But what does fear seek at the end of the ride?" (VOY: "The Thaw")
In her Captain's log in 2373, Captain Kathryn Janeway stated regarding the USS Voyager's recent encounters with the Borg and Species 8472, "The Northwest Passage is only one day away and I won't allow fear to undermine this crew's sense of purpose, even if that fear is justified." (VOY: "Scorpion")
Seven of Nine felt that the Omega Directive was the result of Starfleet's ignorance and fear regarding the omega molecule, but Captain Kathryn Janeway felt that "sometimes fear should be respected." (VOY: "The Omega Directive")