Fax (short for facsimile, from Latin fac simile, "make similar", i.e. "make a copy") is a telecommunications technology used to transfer copies (facsimiles) of documents, especially using affordable devices operating over the telephone network. The word telefax, short for telefacsimile, for "make a copy at a distance", is also used as a synonym. The device is also known as a telecopier in certain industries. When sending documents to people at large distances, faxes have a distinct advantage over postal mail in that the delivery is nearly instantaneous, yet its disadvantages in quality and its proprietary format have relegated it to a position beneath email as the prevailing form of electronic document transferal.

A "fax machine" usually consists of an image scanner, a modem, a printer, and usually a phone combined into a single package. The scanner converts the content printed on a physical document into a digital image, the modem sends the image data over a phone line to another device, and the printer at the far end produces a copy of the transmitted document.

Some fax machines can be connected to a computer, and the individual components -- the scanner, printer, and occasionally the modem -- can be used independently. Such devices are usually called multifunction printers or MFPs. Fax capabilities are also offered as options for many high-volume workgroup printers and photocopiers.

Although devices for transmitting printed documents electrically have existed, in various forms, since the mid to late 19th century (see "History" below), modern fax machines became feasible only in the mid-1970s as the sophistication increased and cost of the three underlying technologies dropped. Digital fax machines first became popular in Japan, where they had a clear advantage over competing technologies like the teleprinter, since at the time (before the development of easy-to-use input method editors) it was faster to handwrite kanji than to type the characters. Over time, faxing gradually became affordable, and by the mid-1980s, fax machines were very popular around the world.

Although many businesses still maintain some kind of fax capability, the technology has faced increasing competition from Internet-based systems. However, fax machines still retain some advantages, particularly in the transmission of sensitive material which, due to mandates like Sarbanes-Oxley and HIPAA, cannot be sent over the Internet unencrypted. In some countries, because digital signatures on contracts are not recognized by law while faxed contracts with copies of signatures are, fax machines enjoy continuing popularity in business.

In many corporate environments, standalone fax machines have been replaced by "fax servers" and other computerized systems capable of receiving and storing incoming faxes electronically, and then routing them to users on paper or via secure email. Such systems have the advantage of reducing costs by eliminating unnecessary printouts and reducing the number of inbound analog phone lines needed by an office.

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