What is the X in X-mas?
early 15c., "harmony, agreement," from Medieval Latin correspondentia, from correspondentem (nominative correspondens), present participle of correspondere (see correspond). Sense of "communication by letters" is first attested 1640s.
correspondence cor·re·spon·dence (kôr'ĭ-spŏn'dəns)
A relationship between corresponding points on each retina such that stimulation produces a single image.