H21 or H-21 may refer to:
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British NVC community H21, a type of
heath community in the British National Vegetation Classification
*
Highway H21 (Ukraine)
H21 is an important Ukraine national highway ( H-highway) in the Donetsk Oblast and Luhansk Oblast of Donbas, Ukraine. It connects Donetsk through Luhansk with Starobilsk. The highway runs both, west–east between Donetsk and Luhansk, and south ...
* , a Royal Navy H-class submarine
* , a Royal Navy S-class destroyer
*
Mata Hari
Margaretha Geertruida MacLeod (née Zelle; 7 August 187615 October 1917), better known by the stage name Mata Hari (), was a Dutch exotic dancer and courtesan who was convicted of being a spy for Germany during World War I. She was executed by ...
(1876–1917), German spy code named H-21
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Piasecki H-21
The Piasecki H-21 Workhorse/Shawnee is an American helicopter, the fourth of a line of tandem rotor helicopters designed and built by Piasecki Helicopter (later Boeing Vertol). Commonly called "the flying banana", it was a multi-mission helicop ...
, an American helicopter
* Institute H21, a Czech think tank that created the website
Prezident 21 Prezident 21 is an interactive website, created by the Czech mathematician and political activist Karel Janeček and his think tank, Institute H21, used to familiarise voters with a proposed system to select a presidential candidate for the 2018 Cze ...
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