|Centuries:||1st century BC – 1st century – 2nd century|
|Decades:||0s BC 0s 10s – 20s – 30s 40s 50s|
|Years:||18 19 20 – 21 – 22 23 24|
|21 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||774|
|Bahá'í calendar||−1823 – −1822|
|English Regnal year||N/A|
|Chinese calendar||庚辰年 (Metal Dragon)
2717 or 2657
— to —
辛巳年 (Metal Snake)
2718 or 2658
|Coptic calendar||−263 – −262|
|- Vikram Samvat||77–78|
|- Shaka Samvat||N/A|
|- Kali Yuga||3122–3123|
|Igbo calendar||−979 – −978|
|Iranian calendar||601 BP – 600 BP|
|Islamic calendar||619 BH – 618 BH|
|Minguo calendar||1891 before ROC
|Thai solar calendar||564|
|Wikimedia Commons has media related to 21.|
Year 21 (XXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Caesar (or, less frequently, year 774 Ab urbe condita). The denomination 21 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Revolt of the Aedui under Julius Florus and Julius Sacrovir, suppressed by Gaius Silius.
- Emperor Tiberius is a Roman Consul for the fourth time.
- The Romans create a buffer state in the territory of the Quadi in southern Slovakia.
- Barracks are constructed for the Praetorian Guard on the Quirinal.
- King Daeso of Dongbuyeo is killed in battle against the armies of Goguryeo, led by its third ruler, King Daemusin.
Arts and sciences
- The manufacture of pens and metal writing tools begins in Rome.
- Publius Sulpicius Quirinius, Roman governor of Syria (b. c. 51 BC)
- Marcus Valerius Messalla Barbatus, Roman Consul (or 20) (b. 11 BC)
- Arminius, Germanic leader who defeated three Roman legions at the Battle of the Teutoburg Forest (b. 18/17 BC)
- Empress Wang
- King Daeso of Dongbuyeo (b. 60 BC)