void variable problem

[Haines Brown]

I installed synonyms.el, and it depends on access to mthesar.txt. So I
created: /home/haines/mthesaur/mthesaur.txt and in .emacs access it with
the line (setq synonyms-file  /home/haines/mthesaur/mthesaur.txt)

But when emacs loads, I get this error:

  Debugger entered--Lisp error: (void-variable
       /home/haines/mthesaur/mthesaur\.txt) 
    (setq synonyms-file /home/haines/mthesaur/mthesaur\.txt)
    eval-buffer(#<buffer  *load*<4>> nil "/home/haines/.emacs" nil t)  ; 
      Reading at buffer position 17897
    load-with-code-conversion("/home/haines/.emacs"
       "/home/haines/.emacs" nil nil) 
    load("~/.emacs")

What does the backshash in "mthesaur\.txt" mean?

Haines Brown