It's a specific phobia, a type of anxiety disorder characterized by an intense and irrational fear of something specific.
Here's how to break down the word:
* Triskaideka: Comes from the Greek words "treis" (three) and "deka" (ten), meaning "thirteen".
* Phobia: From the Greek word "phobos" meaning "fear".
So, triskaidekaphobia literally translates to "fear of thirteen".