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|251 by topic|
|Ab urbe condita||1004|
|Balinese saka calendar||172–173|
|Chinese calendar||庚午年 (Metal Horse)|
2947 or 2887
— to —
辛未年 (Metal Goat)
2948 or 2888
|Coptic calendar||−33 – −32|
|- Vikram Samvat||307–308|
|- Shaka Samvat||172–173|
|- Kali Yuga||3351–3352|
|Iranian calendar||371 BP – 370 BP|
|Islamic calendar||382 BH – 381 BH|
|Minguo calendar||1661 before ROC|
|Seleucid era||562/563 AG|
|Thai solar calendar||793–794|
377 or −4 or −776
— to —
378 or −3 or −775
Year 251 (CCLI) 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 Traianus and Etruscus (or, less frequently, year 1004 Ab urbe condita). The denomination 251 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.
- July 1 – In the Battle of Abritus, the Goths defeat emperor Decius and his son Herennius Etruscus on swampy ground in the Dobruja (Moesia).
- In Rome, Hostilian, son of Decius, succeeds his father, while Trebonianus Gallus is proclaimed Emperor by the troops. Gallus accepts him as co-emperor, but an outbreak of plague strikes the city and kills the young Hostilian.
- The prosperity of Roman Britain declines during this period as the Germanic tribes of the Franks and Saxons, whose homelands are in Friesland and the Low Countries, make raids around the southeast coast.
- Gallus makes peace with the Goths, he permits them to keep their plunder, and offers them a bribe not to return.
- A fifteen-year plague begins in the Roman Empire.
- The Sassanid king, Shapur I, orders an invasion of the Roman East with the intent of finally capturing the jewel of Syria, Antioch (251–254).
- Wang Ling's rebellion against the Wei regent Sima Yi is quelled.
- Sima Yi passes away in Luoyang.
- Sima Shi, Sima Yi's eldest son, inherits his father's authority.
- Saint Agatha
- July – August
- Wang Ling, Chinese general
- September 7 – Sima Yi, Chinese general, statesman and regent of the Cao Wei state (b. 179)
- Deng Zhi, Chinese general, official and diplomat of the Shu Han state
- Empress Zhen, Chinese empress of the Cao Wei state
- Hostilian, Roman emperor (of plague in Rome)
- Origen (possible date)