Fear

Fear was an emotion.

Some beings could feed on fear, like the entity on Phobos (AUDIO: Phobos) and the Fearmonger. (AUDIO: The Fearmonger)

The Keller Machine fed off the fears of its victims. They included Arthur Linwood, who was afraid of rats; Kettering, who was afraid of drowning; the Third Doctor, who was afraid of fire; Alcott, who was afraid of the Chinese; and, who was afraid of the Doctor. (TV: The Mind of Evil)

Permians hunted by inducing a state of fear in the minds of their prey. (AUDIO: The Land of the Dead)

The Sixth Doctor listed vertigo with agoraphobia and xenophobia as fears which, when mixed together, would form "naked fear". (PROSE: The Ultimate Evil)

George psychically deposited all his fears in his doll's house, which manifested as peg dolls. (TV: Night Terrors)

On the Minotaur's prison ship were pictures of those who had fed it and what their fear was. They included Novice Prin, who was afraid of sabrewolves; Lady Silver-Tear, who was afraid of Daleks; Royston Luke Gold, who was afraid of Plymouth; Tim Heath, who was afraid of having his picture taken and Tim Nelson, who was afraid of balloons. (TV: The God Complex)

According to Miss Quill, the "first fear" was a fear that "everyone" went back to and could not face. The Lorr Ballon claimed that this was just cowardice. Quill claimed that a soldier without fear was inefficient, and was one that won battles, but lost wars. (TV: The Metaphysical Engine, or What Quill Did)