It is hard to say in TAB as the symbols are not standardized.
A number of sources refer to ghost notes. A ghost note is a musical note with a rhythmic value, but no discernible pitch when played. It is more of a percussive accent than a note. Lightly put your finger on the string and pick it, you will get more of a click than a note. Lightly muted, as Christopher described. If you do a youtube search for ghost notes you can see a number of examples.
Here is one:
-Elliot
