Triskaidekaphobia

Since this is Friday the 13th, here's a word you should know:

Main Entry: tris·kai·deka·pho·bia
Pronunciation: "tris-"kI-"de-k&-'fO-bE-&, "tris-k&-
Function: noun
Etymology: New Latin, from Greek treiskaideka thirteen (from treis three + kai and + deka ten) + New Latin phobia -- more at THREE, TEN
: fear of the number 13

Source: Merriam-Webster's Online Dictionary.