Reciprocity effect refers to the relationship between exposure time and light intensity on a photograph. The idea, and fact of the matter is, that an exposure is the fruit of both factors (exposure time and light intensity), not either individually.

A failure of reciprocity occurs when an exposure is made outside of the range of a film stocks intended exposure time (i.e. the exposure time is too long or too short). What results is either a shift in the color balance, inaccurate contrast and/or a loss of the effective film speed or ISO.