123 (one hundred [and] twenty-three) is the natural number following 122 and preceding 124.

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• 123 is a Lucas number.^[1] It is the eleventh member of the Mian–Chowla sequence.^[2]
• Along with 6, 123 is one of only two positive integers that is simultaneously two more than a perfect square and two less than a perfect cube (123 = 11² + 2 = 5³ - 2).^[3]
• 123 is the first whole number containing numbers from 1 to 3.

The Book of Numbers says that Aaron died at the age of 123.^[4]

In numerology, the sequence of 123 is associated with progress and the Holy Trinity, so it may be referred to as the Trinity of Progress or Triad of Progress.^[5]

• The emergency telephone number in Colombia
• The telephone number of the speaking clock for the correct time in the United Kingdom
• The electricity (PLN) emergency telephone number in Indonesia
• The medical emergency telephone number in Egypt
• The Notation for national and international telephone numbers Recommendation ITU-T Recommendation E.123 defines a standard way to write telephone numbers, e-mail addresses, and web addresses

123 is also:

• 123 (film), a 2002 Indian film
• 123 (interbank network), shared cash network in Egypt
• 123 (New Jersey bus)
• "1-2-3", 1965 song written and recorded by Len Barry
• The atomic number of the yet-to-be-discovered element unbitrium

• 1-2-3 (disambiguation)
• 123rd (disambiguation)
• AD 123
• 123 BC
• List of highways numbered 123
• Section 123 Agreement of the U.S. Atomic Energy Act of 1954
• United Nations Security Council Resolution 123
• Japan Airlines Flight 123, world's deadliest single-aircraft accident in history
• Raz, Dwa, Trzy, Polish music band
• Raz, dwa, trzy (newspaper), Polish sports weekly
• Sonnet 123 by William Shakespeare
• The Lotus 1-2-3 spreadsheet program