Hexakosioihexekontahexaphobia (literally, “fear of the number six-hundred-sixty-six”) is the fear that originates in the Biblical verse Revelation 13:18 which indicates that the number 666 is the Number of the Beast, linked to Satan or the Anti-Christ. Outside the Christian faith, the phobia has been further popularized as a leitmotif in various horror films.

Hexakosioihexekontahexaphobes will strongly avoid things related to the number 666. A prominent example is of Nancy and Ronald Reagan who, in 1989, when moving to their home in the Bel-Air section of Los Angeles, had the address of 666 St. Cloud Road changed to 668 St. Cloud Road. Some women also expressed concern on giving birth to a child on 6/6/2006.

Rarer is trying to avoid the number as it relates to other numbers. For example, the fraction 2/3 has a repeating decimal of .666…. A severe hexakosioihexekontahexaphobe may avoid all fractions that would include this recurring decimal. Those with hexakosioihexekontahexaphobia may consider it bad luck to get three sixes in a hand of poker, even though this is usually quite a good hand.

Many are afraid of the digits 666, rather than the quantitative number adding up to this amount.

Source: Wikipedia