**666** (*six hundred sixty-six*) is the natural number following 665 and preceding 667.

Six hundred and sixty-six is called the "number of the Beast" in (most manuscripts of) chapter 13 of the Book of Revelation, of the New Testament, and also in popular culture.

666 is the sum of the first 36 natural numbers (
∑
i
=
1
36
i
, i.e. 1 + 2 + 3 + ... + 34 + 35 + 36 = 666), and thus it is a triangular number. Notice that 36 = 15 + 21; 15 and 21 are also triangular numbers; and 15^{2} + 21^{2} = 225 + 441 = 666.

In base 10, 666 is a repdigit (and therefore a palindromic number) and a Smith number. A prime reciprocal magic square based on 1/149 in base 10 has a magic total of 666.

The prime factorization of 666 is 2 • 3^{2} • 37. Also, 666 is the sum of squares of first seven primes:
2
2
+
3
2
+
5
2
+
7
2
+
11
2
+
13
2
+
17
2

The Roman numeral for 666, DCLXVI, has exactly one occurrence of all symbols whose value is less than 1000 in decreasing order (D = 500, C = 100, L = 50, X = 10, V = 5, I = 1).

In the Textus Receptus manuscripts of the New Testament, the Book of Revelation (13:17–18) cryptically asserts 666 to be "man's number" or "the number of a man" (depending on how the text is translated) associated with the Beast, an antagonistic creature that appears briefly about two-thirds into the apocalyptic vision. Some manuscripts of the original Greek use the symbols χξϛ chi xi stigma (or ϝ digamma), while other manuscripts spell out the number in words.

In modern popular culture, 666 has become one of the most widely recognized symbols for the Antichrist or, alternatively, the devil. The number 666 is purportedly used to invoke Satan. Earnest references to the number occur both among apocalypticist Christian groups and in explicitly anti-Christian subcultures. References in contemporary Western art or literature are, more likely than not, intentional references to *the Beast* symbolism. Such popular references are therefore too numerous to list.

It is common to see the symbolic role of the *integer* 666 transferred to the *digit sequence* 6-6-6. Some people take the Satanic associations of 666 so seriously that they actively avoid things related to 666 or the digits 6-6-6. This is known as hexakosioihexekontahexaphobia.

In some early biblical manuscripts, including Papyrus 115, the number is cited as 616.

In the Bible, 666 is the number of talents of gold Solomon collected each year (see 1 Kings 10:14, 2 Chronicles 9:13 and also in Ezra 2:13).
In the Bible, 666 is the number of Adonikam's descendants who return to Jerusalem and Judah from the Babylonian exile (see Ezra 2:13).
In the Bible, there may be a latent reference to 666 in the name of the great sixth-century BC king of Babylon. Commonly spelled *Nebuchadnezzar*, transliterating from the Book of Daniel, the name is *Nebuchadrezzar* or *Nebuchadrezzur* in the Book of Jeremiah (see Jeremiah 49:28–30). The number of each name can be calculated, since Hebrew letters double as numbers (see Gematria, Hebrew numerals). *Nebuchadrezzar* is 663, and *Nebuchadrezzur*, 669. Midway between the two variants is 666. If the mysteries of Jeremiah are to be related to those of Revelation, Nebuchadrezzar, who came (though bidden by God) to crush God's people, may prefigure the end-times beast.
Using gematria, Neron Caesar transliterated from Greek into Hebrew produces the number 666. The Latin spelling of "Nero Caesar" transliterated into Hebrew produces the number 616. Thus, in the Bible, 666 may have been a coded reference to Nero the Roman Emperor from 55 to 68 AD.
Is the magic sum, or sum of the magic constants of a six by six magic square, any row or column of which adds up to 111.
Is the sum of all the numbers on a roulette wheel (0 through 36).
Was a winning lottery number in the 1980 Pennsylvania Lottery scandal, in which equipment was tampered to favor a 4 or 6 as each of the three individual random digits.
Was the original name of the Macintosh SevenDust computer virus that was discovered in 1998.
The number is a frequent visual element of Aryan Brotherhood tattoos.
Aleister Crowley adopted the title "the Great Beast 666". As such, 666 is also associated with him, his work, and his religious philosophy of Thelema.
Molar mass of the high-temperature superconductor YBa_{2}Cu_{3}O_{7}
In China the number is considered to be lucky and is often displayed in shop windows and neon signs. In China, 666 can mean "everything goes smoothly".