Irrational
fear (
phobia) of the number
thirteen (from the Greek
triskaideka - thirteen - and
phobos, meaning fear), most often for reasons of cross-cultural
superstition ranging from the number of attendees at
the Last Supper to
Apollo 13th's (launched at 13:13 Central Time) oxygen tank explosion on the 13th of April to the number (
13) of months (each of 28 days) in the calendar of the (pagan)
lunar year.
Manifests itself contemporarily in dread of sporadic (yet frequent) Friday the 13ths, cessation of learning times tables at "12 x 12", and the omission of the 13th floor from most skyscrapers and office blocks, whose elevators will quite glibly skip from 12 to 14 without ever suggesting that anything else might conceivably be found between them, many hotel room and airplane layouts similarly lacking a room or seat #13. Many street addresses that would otherwise end in 13 instead have amended signs indicating that they are instead 12 and a half, Italy takes precautions against the number 13 appearing in state lottery draws, and what we know as a baker's dozen is in Scotland apparently frequently known as "the Devil's dozen."
Triskaidekaphobia is often erroneously spelt triskadekaphobia, especially when sampled into pop culture, perhaps on account of the i-less pronunciation of the word.