Terms and Definitions

Accelerando - with gradual increase of speed.

Ritardando - with gradual decrease of speed

Fermata - a pause of unspecified length on a note or rest.

Caesura - Break or interruption in music, notated by two diagonal lines often referred to as railroad tracks. The break can be of any length at the discretion of the conductor.

Breath mark - 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.

http://www.classicalworks.com/html/glossary.html