X21 (disambiguation)
X21 may refer to:
- Northrop X-21, experimental aircraft designed to test wings with laminar flow control
- SA-X-21 or S-400 (missile), anti-aircraft weapon system developed by Russia's Almaz Central Design Bureau as an upgrade of the S-300 family
- X.21 (sometimes referred to as X21), an interface specification for differential communications introduced in the mid-1970s by the ITU-T
- X21 (band) (次世代ユニット), a Japanese girl idol group
This disambiguation page lists articles associated with the same title formed as a letter–number combination.
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