A genius is an individual with an exceptional capacity for knowledge and intelligence. Geniuses can appear in a variety of forms and with a variety of specialties, and their exceptional intelligence is not necessarily universal, potentially only applying to certain subjects and fields that they excel in. These can be any number of things, such as scientific fields, military tactics, martial arts, and/or investigation. There are many levels of genius, as well, detailed on the intelligence page; a regular genius, on the level of those that exist in the real world, is generally limited to a few areas of expertise, while fictional individuals of greater intellects may be masters of many, if not all fields of science and even strategy as well.
Geniuses: L and Light Yagami (Death Note), Solid Snake (Metal Gear), Shikamaru Nara and Kakashi Hatake (Naruto), Colress (Pokémon), Sherlock Holmes (Sherlock Holmes), Kurama and Yū Kaitō (Yu Yu Hakusho)
Extraordinary Geniuses: Kisuke Urahara and Sosuke Aizen (Bleach), Lelouch vi Britannia (Code Geass), Batman (DC Comics), Bulma, Doctor Gero, and Android 21 (Dragon Ball), Joseph Joestar and the Pillar Men (JoJo's Bizarre Adventure), Spider-Man, Bruce Banner, and Iron Man (Marvel Comics), Night of Wallachia and Dust of Osiris (Melty Blood), Ozymandias (Watchmen)
Supergeniuses: Brainiac (DC Comics), The Doctor (Doctor Who), Doctor Doom, Mister Fantastic, Thanos, and the High Evolutionary (Marvel Comics), Rick Sanchez (Rick & Morty), Washu Hakubi (Tenchi Muyo!), The Emperor of Mankind (Warhammer 40,000)