Definition and background:
A directive to the performer to break the phrase at that point in the composition and breathe, thus assisting in the production of a smooth phrase consistent with the composer's wishes. The breath mark looks like a large comma or apostrophe and should always be located at the end of the phrase above the staff. Also [Fr.] pauses, [Ger.] Luftpausen, [It.] pause. In a score, this symbol tells the performer to take a breath. (or make a slight pause for non-wind instruments). This pause usually does not affect the overall tempo. For bowed instruments, it indicates to lift the bow and play the next note with a downward bow.
For more information on the music term "Breath mark" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary