## restricted command found (def)

As part of a LaTeX expresion, I have \text{defined by}, and the error is 'restricted command found (def)' which is apparently reffering to the defined by. I did a search and found this thread on MHF:

http://www.mathhelpforum.com/math-he...g-latex-6.html

So why doesn't it work? why is there a problem with "name" and the 'def' part of "defined by"? Thats a bit rubbish isn't it?