U21 or U-21 may refer to:
Naval vessels
* , various vessels
* , a sloop of the Indian Navy
* , a submarine of the Austro-Hungarian Navy
Other uses
*
Beechcraft U-21 Ute, an American utility aircraft
*
Great rhombihexahedron
In geometry, the great rhombihexahedron (or great rhombicube) is a nonconvex uniform polyhedron, indexed as U21. It has 18 faces (12 squares and 6 octagrams), 48 edges, and 24 vertices. Its dual is the great rhombihexacron. Its vertex figure ...
*
Small nucleolar RNA SNORD21
*
Universitas 21
Universitas 21 (U21) is an international network of research-intensive universities. Founded in Melbourne, Australia in 1997 with 11 members, it has grown to include twenty-eight member universities in nineteen countries and territories.
The uni ...
, an international network of research-intensive universities
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Uppland Runic Inscription 21
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