noun

• (soccer) An especially devoted football fan, typically associated with the intimidating use of extremist slogans, pyrotechnics and sometimes hooligan violence.
  "A similar view is expressed by a Turin supporter in Segre's study, but in this case it is more specifically addressed to how powerful teams, such as Juventus, get preferential treatment in reports on the negative aspects of the ultras world."
• (athletics) An ultramarathon.
  "I've done more than forty marathons and ultras and have won a handful of small boutiquey races in mountainous, out-of-the-way places: the foothills of the Sierra Nevada; Mount Mitchell, North Carolina; Bozeman, Montana; and, on the third day of a 100-mile stage race, the Mount Everest Challenge Marathon in the Himalayas."
• (climbing) An ultra-prominent peak.
  "Blanca Peak is one of just three ultra-prominence peaks, or “ultras,” in the state and the highpoint of the Sierra Blanca Range, a massif that includes ranked 14ers Ellingwood Point, Little Bear Peak, and Mount Lindsey."
• (usually capitalized) Code name used by British codebreakers during World War 2 for decrypted information gained from the enemy.