Xanadu  One name for a fantasy Utopia.

Xanth   A fantasy world created by Piers Anthony.  Sounds like it would be a fun place to visit.

Xenophobia  The fear of aliens.

Xerox   A company that made a big name for itself by making machines that copy pieces of paper.

X-Files   TV show.  Premise is an FBI department involved in "unexplained" phenomena.

X-Rated  A descriptor for something that is only for adults.  Usually refers to movies.  In America, it usually applies to sex.  In Europe, it usually applies to violence.

X-Ray A type of radiation that is used to see "inside" something. Originally discovered with a radioactive source and film, modern day images, usually called radiographs are mostly artificially generated.