Reciprocity Failure

Exposure 101 explained the relationship between aperture and shutter speed.  You'll recall that selecting the right combination of aperture and shutter speed generally determines how much light reaches the film and that there is a linear relationship between the two.  For example, closing the aperture one stop requires that the shutter speed be one stop longer.  This linear relationship is called "reciprocity".

However, this linear relationship does break down when very long shutter speeds are used.  This "breakdown" is called "reciprocity failure."

Basically, when shutter speeds are about 1 second or longer, some additional exposure becomes required.  For example, if you meter a scene and determine that you need a shutter speed of 4 seconds, you may need to add an extra 1/2 to 1 stops (ie shoot at 6 or 8 seconds instead).  The exact amount depends on what kind of film you are using.

For example, the reciprocity failure compensations for Fuji Velvia are:
 

Kodak offers advice on its slide films here.
Fuji offers advice on its slide films here.