Originally Posted by: Azraelsometimes it is quite usefull to use google

http://www.guitarnoise.com/faq.php?id=44

"The best description of a ghost note, is a note that is felt but not heard. You will play the note softer, and without emphasis."

I have to disagree with this quote from the Google link you posted.

A ghost note is not always played "softer" and "without emphasis".

A ghost note is used primarily for rhythmic contribution and therefore is sometimes played just as loud or louder (accented) than the melodic or harmonic line that you are embellishing.

This is not to say that a ghost note cannot be played softly.