What is correspondence?

More definitions of "correspondence":

• (noun): (mathematics) an attribute of a shape or relation; exact correspondence of form on opposite sides of a dividing line or plane.
  Synonyms: symmetry, symmetricalness, balance

• (noun): The relation of correspondence in degree or size or amount.
  Synonyms: commensurateness, proportionateness

• (noun): A function such that for every element of one set there is a unique element of another set.
  Synonyms: mapping, map

• (noun): Compatibility of observations.
  Example: "The results of two tests were in correspondence"
  Synonyms: agreement

• (noun): Communication by the exchange of letters.

• (noun): Similarity by virtue of correspondence.
  Synonyms: parallelism