Correspondence means communication between people through letters, emails, or other written messages. When you write to your grandmother and she writes back, you're engaged in correspondence with her. When a scientist exchanges emails with researchers around the world about a discovery, that's professional correspondence.

The word comes from the idea that messages correspond to each other: they match up, reply to previous points, and create an ongoing conversation. Think of correspondence as a two-way street of written communication. A single letter isn't really correspondence, but when you write back and forth with someone over time, you build up a correspondence.

In earlier centuries, correspondence was crucial for sharing ideas across distances. Benjamin Franklin's correspondence with other scientists helped spread knowledge about electricity. Abraham Lincoln's correspondence during the Civil War helped him lead the nation through crisis. Today, email correspondence happens instantly, but the basic idea remains the same: people exchanging thoughts and information through writing.

The word can also mean a matching relationship between things. In mathematics, teachers talk about one-to-one correspondence when each item in one group pairs with exactly one item in another group, like matching socks in a pile. When detectives notice correspondence between different pieces of evidence, they mean the clues match up or align in meaningful ways.