Crescendo is increasing volume (from the Italian for 'growing'). It is indicated by a rightward opening horizontal wedge or the abbreviation cresc.

 

It can happen quickly, as when a performer plays a single note and makes it suddenly louder, or it can happen gradually, as when volume increases incrementally over many bars.

 

Decrescendo, or diminuendo, is decreasing tone volume and is indicated by a leftward opening horizontal wedge or the abbreviation decresc or dim.