An inbox is the place where incoming messages arrive and wait for you to read them. Originally, an inbox was a physical tray on someone's desk where assistants would place letters, memos, and documents that needed attention.

Today, when most people say inbox, they mean their email inbox: the screen or folder where new emails appear. When you check your inbox in the morning, you're looking to see what messages arrived overnight. A full inbox means you have lots of unread emails waiting. Some people feel satisfied when they achieve “inbox zero,” meaning they've read, responded to, or filed everything.

The word can apply to any system where messages collect. You might have an inbox for text messages on your phone, or an inbox in a classroom app where your teacher sends assignments. Social media apps often have inboxes for private messages.

The opposite of an inbox is an outbox, where outgoing messages wait to be sent, though this term is less common now since many messages are sent instantly. When something lands in your inbox, you usually need to deal with it, whether that means replying, deleting it, or saving it for later.