In music notation, a grace note (occasionally called an acciaccatura) is a type of ornamentation drawn as small eighth note with a slash through its stem and flag positioned just before a note, either a half step or a whole step higher on the staff. It is not factored in when calculating the time signature for the current measure.

A grace note is played on the beat as a very, very short note slurred into the note it precedes, typically the length of a thirty-second note which is "stolen" from the main note. On the staff, it looks something like this (complete with time signature):

    /\                                                     
---| /------------------------|------------------|-------
   |/                         |     |            |        
---/-----------*-------|X-----|-----|------------|-------
  /|     2    |       .       |     |       |    |        
-/-|/\--------|----------*----|-----|-------|----|-------
|  |  |  4    |         |     |    *     |X |    |       
|--|--|-------|---------|-----|---------.---|----|-------
 \ |  |                 |     |            *     |        
--\|_/------------------|-----|------------------|-------
   |                                                       
  \|