Triskaidekaphobia

Take the word apart and it explains itself. Greek treis is three, kai is and, deka is ten, and -phobia is fear, so the whole thing reads as fear of three-and-ten. Etymology dictionaries date the first English use to 1908, alongside the variant spelling triskaidecaphobia. That makes the label younger than the lift shafts most people associate it with, and a great deal younger than the unease it names.

What the word denotes in ordinary use is a cultural avoidance rather than a clinical condition. A hotel that omits floor 13, or a diner who will not sit thirteenth at a table, is following a convention, and almost nobody involved reports the distress a diagnosed specific phobia requires. Number-focused anxiety of the clinical kind exists and is rare. The everyday word covers the custom.

Friday the thirteenth has a longer word of its own, paraskevidekatriaphobia, from Greek paraskevi for Friday, dekatreis for thirteen and the same -phobia ending. The origin stories, the documented history of skipped floors and airline seat rows, and the research on whether the date is genuinely dangerous are set out on the unlucky numbers page. One figure travels with the topic and is worth flagging here. The widely repeated claim that 85% of tall buildings have no floor labelled 13 comes from an Otis Elevator spokesman quoted in press coverage, drawing on the company's own installation records. Otis has never published that review, and the number circulates in several mutually inconsistent forms.