What is “fax” short for?

Fax is short for “facsimile” when used in the context of electronically transmitting a document between fax machines.

The official definition of “facsimile” is a copy or reproduction of an item (typically a work of art or a document).

A reproduced work of art is called a facsimile, as is a reproduced map. Neither of these are called faxes.

A document electronically transmitted between fax machines is called a fax. It can also be called a facsimile.

Sources and more information:

Wikipedia – Fax

Dictionary.com – Facsimile and Fax

Adams on Contract Drafting – A Gripping Issue of Notice-Provision Technology: “Telecopier,” “Facsimile,” or “Fax”?