A fax (short for “facsimile”) is a machine that sends copies of paper documents through telephone lines to another fax machine, which prints them out. Before email became common in the 1990s, fax machines were the fastest way to send a written document across long distances. If your dad needed to send a signed contract from New York to Los Angeles, he could feed it into a fax machine, dial the recipient's fax number, and within minutes an exact copy would print out 3,000 miles away.

The technology works by scanning a document and converting it into electronic signals that travel through phone lines, then reconstructing and printing the image on the other end. It was revolutionary when it became popular in the 1980s: suddenly people could send drawings, signatures, forms, and printed pages almost instantly instead of waiting days for mail delivery.

Today, fax machines seem outdated since we can email documents or send photos instantly from our phones. But some businesses, especially medical offices and law firms, still use faxes because the technology has certain legal protections and security features.

As a verb, to fax something means to send it this way. When someone says they'll “fax it over,” they mean they'll send a document this old-fashioned but reliable way.