The most concise definition of eldritch can be described in a word: weird or eerie. Its origin, however, is much more fun.
Eldritch was first used in 1508 and is thought to originate from the Middle English word elfriche, the combination of "elf" and the Old English "rice" or "riche," meaning kingdom. So, loosely translated, eldritch is referring to the elf kingdom or, its more common association, fairyland.
For fans of H.P. Lovecraft, the term is instantly recognizable as a macabre word used throughout his dark stories. A collection of his short fiction is contained in the book Eldritch Tales.
I think it's a fascinating word and one I wouldn't mind using in a story myself. What do you think? Are there other definitions or examples of eldritch magic that you've come across in fictional worlds?