title: X21 (disambiguation)
text: # X21 (disambiguation)
  
  X21 may refer to:
  
  * Northrop X-21, an experimental aircraft
  * SA-X-21 or S-400 (missile), a Russian anti-aircraft weapon system
  * X.21 (sometimes referred to as X21), an interface specification for differential communications
  * X21 (band) (次世代ユニット), a Japanese girl idol group
  