Emotion or feeling was a response to an object or a situation that induced either a negative or positive mental state. Being able to identify and recognise other's emotions was called empathy.
Although most species felt at least some kinds of emotions, others intentionally did not. Considering them to be "weaknesses," the Daleks were deliberately designed by Davros to have no emotions, (TV: Genesis of the Daleks) while the Cybermen removed theirs during cyber-conversion, (TV: The Tenth Planet) some Cybermen using emotional inhibitors to do so. (TV: The Age of Steel) However, Missy explained that while Cybermen suppressed emotion, Daleks actually channeled them, using hate and rage and fear to exterminate everything that was not a Dalek. (TV: The Witch's Familiar) As such, some Cybermen could act emotional, such as one Cyber-Leader who used the phrase "excellent" and whom the Fifth Doctor considered "flippant," (TV: Earthshock) as well as the Cyberman Kroton who was able to regain his emotions. (COMIC: Throwback: The Soul of a Cyberman)
In some societies, emotion was forbidden. The people of the city of Zombos on the planet Zom had their emotions removed on the orders of the Brains Trust. (COMIC: City of the Damned) The Gråttites banned emotion due to the threat of the Mörkön. (COMIC: The Whispering Gallery)
Some species fed on emotion, or certain types of emotion, for sustenance. These included Cerebravores, (COMIC: Revolutions of Terror), Empathivores, (COMIC: The Whispering Gallery) the Fearmonger, (AUDIO: The Fearmonger) the Ilk, (AUDIO: The Forever Trap) the Isolus, (TV: Fear Her) Kharitites, (COMIC: After Life) the Lankin, (TV: Nightvisiting) the Lapino, (PROSE: Planet of the Bunnoids) the Lobri, (COMIC: Ground Zero) Pranavores, (COMIC: Revolutions of Terror) Psionovores, (AUDIO: Minuet in Hell) the Qetesh, (TV: Goodbye, Sarah Jane Smith) the Scourge, (AUDIO: The Shadow of the Scourge) the Shroud, (PROSE: Shroud of Sorrow) the Threckon, (PROSE: Inmate 280) and the Mara which fed on raw emotion. (PROSE: The Left-Handed Hummingbird)
Emotions were also extremely powerful, at least in human cases, and could be harnessed as weapons. An emotional surge from six billion people all at once was enough to cripple a Qetesh and destroy her stomach. (TV: Goodbye, Sarah Jane Smith) Emotions were also lethal to Cybermen, causing at least one's head to explode. (TV: The Age of Steel)