Correspondence (noun)

1. Communication by exchange of letters.
2. Agreement or consistency between two or more things.
3. A relationship of similarity or equivalence between two or more things.

Origin:

Late middle english: from old french correspondance, from correspondre 'be in agreement', from latin correspondere, from con- 'together' + respondere 'respond'.

Examples:

1. She maintained a regular correspondence with her friends and family.
2. There was a close correspondence between the results of the two experiments.
3. The correspondence between the map and the actual terrain was remarkable.
4. There was a correspondence between the symptoms described in the medical records and the patient's current condition.
5. The correspondence course allowed her to study at her own pace and convenience.