/ˌkɒrɪˈspɒnd/
[verb] To match or be similar to something else, or to communicate regularly with someone by writing letters or messages. When two things correspond, they fit together or are related in a clear way.
Before emails and telephones, 'correspond' was essentially the only word the educated world had for staying in touch across distances.
The word 'correspond' travels to us along a well-worn Latin road, beginning with the prefix 'com-', meaning 'together' or 'with', and the verb 'respondere', meaning 'to answer' or 'to promise in return'. That Latin verb itself was built from 're-', meaning 'back', and 'spondere', meaning 'to pledge' or 'to make a solemn promise'. So buried deep inside 'correspond' is the ancient idea of two parties making promises to each other — a kind of sacred exchange.
From Latin, the word moved into Medieval Latin as 'correspondere', where scholars and churchmen used it to describe things that matched or answered one another in a formal, almost logical sense. It then passed into Middle French as 'correspondre', carrying the same sense of mutual agreement and symmetry. English borrowed it from French in the mid-16th century, and early users were mostly philosophers and scientists who needed a word for 'things that match or mirror each other'.
The meaning shifted in a fascinating direction during the 17th century. As letter-writing became central to European intellectual and social life, 'correspond' took on its now-familiar sense of exchanging letters with someone. Educated men and women would 'correspond' with friends, scholars, and lovers across countries and continents. The great thinkers of the Enlightenment — Voltaire, Locke, Newton — maintained enormous networks of correspondents. The word had moved from a cold, logical matching to something warm and human: the act of reaching across distance.
The related noun 'correspondence' appeared in English at almost the same time, and the agent noun 'correspondent' followed closely behind. Today a 'foreign correspondent' is a journalist who reports from abroad — still, in a sense, someone answering back from a distance. The word 'respond' is the direct sibling of 'correspond', sharing the same Latin parent 'respondere', but it never developed the letter-writing sense. Meanwhile, 'sponsor' comes from the very same root 'spondere' — meaning that a modern corporate sponsor and an old-fashioned pen pal are, at their deepest etymological level, both making a kind of pledge.
What makes 'correspond' especially interesting is how neatly its two modern meanings — to match, and to write letters — actually share a single logic. When you write to someone regularly, you are in a sense 'answering' them, keeping pace with them, staying in step. The philosopher's precise meaning and the letter-writer's social meaning are not so far apart after all. Both describe a relationship of mutual response, a back-and-forth that creates connection. The word holds both ideas in perfect balance.