An identifier is a name, number, or code that distinguishes one thing from all others. Your student ID number is an identifier that separates you from every other student in your school. Your email address is an identifier that ensures messages reach you and nobody else.

Identifiers work like labels on moving boxes: they tell you exactly which box contains what. In computer programming, an identifier names a variable, function, or piece of data so the program knows which one to use. A library book's call number is an identifier that helps you find that exact book among thousands of others.

The key feature of an identifier is uniqueness. Two students might share the same first name, but they can't share the same student ID. That's why identifiers often use numbers or combinations of letters and numbers: they reduce the chance of confusion. Your social security number, your passport number, and your library card number are all identifiers designed to point to you specifically, so they should be kept private.

Scientists use identifiers to track everything from specimens in a lab to stars in the sky. Without identifiers, imagine trying to find a specific book in a library where every book just said “Book” on the spine, or trying to call a friend when everyone had the same phone number.