Gastroesophageal reflux disease (GERD), also known as acid reflux, is a long-term condition where stomach contents come back up into the esophagus resulting in either symptoms or complications.
A replacement for the South Korean K200-series infantry fighting vehicles, formerly designated as K300 or XK21 KNIFV (Korea Next-generation Infantry Fighting Vehicle), is currently called the K21.
The ASK 21 is a glass-reinforced plastic (GRP) two-seater mid-wing glider aircraft with a T-tail.
Soviet submarine K-21 was a K-class submarine of the Soviet Navy during World War II.
In geometry, a uniform k21 polytope is a polytope in k + 4 dimensions constructed from the ''E''''n'' Coxeter group, and having only regular polytope facets.
"Va, dal furor portata", K. 21 / K6 19c, is an early concert aria in C major for tenor and orchestra by Wolfgang Amadeus Mozart.