A telegraph is a machine which is used for transmitting messages in the form of electrical impulses which can be converted into data. A message sent via telegraph is called a telegram or cablegram, while someone who operates a telegraph machine is known as a telegrapher. Telegraphy was a major mode of communication from the middle of the 1800s until well into the 1900s, before ultimately being supplanted by inventions like the telephone and the Internet.

The earliest version of the telegraph was developed in the late 1700s, primarily as a thought exercise. This early draft only existed on paper, but it laid the groundwork for various incarnations of the telegraph which appeared in the early 1800s. With the development of the electromagnet, Samuel Morse and Alfred Vail were able to develop and patent a reliable electric telegraph system in 1837.

Morse is often credited with being the inventor of the telegraph, but this is not the case. Numerous other inventors had patented various versions of the telegraph before Morse, and history strongly suggests that Alfred Vail was the scientific brains of the operation. Morse popularized the telegraph, however, and developed a workable, easily learned alphabet which could be transmitted via telegraph.

Originally, telegraph machines had to be connected through a series of wires in order to exchange messages. The operator would key a message in the Morse alphabet, and the receiving telegraph machine on the other side would register the message in the form of clicks made by a bar which struck another bar. By listening to the pattern of clicks, the receiving operator could hear the message and transcribe it before passing it on to the recipient.

In the late 1800s, wireless telegraphy began to emerge, and telegraph messages were transmitted over the radio waves. This marked a drastic change in the system, allowing people to rapidly transmit messages in areas without telegraph cables, and enabling things like ship to ship communication. Wireless telegraphy or radiotelegraphy also laid the groundwork for later methods of communication.

The telegraph is largely obsolete now. One famous telegraph company, Western Union, sent its last telegram in 2006, and numerous other companies have stopped offering telegram services, because consumer demand has fallen radically. Telegrams are generally regarded as interesting curiosities, as is the peculiar language used in telegrams; because transmitting the signal is painstaking, telegraph operators developed their own shorthand to make transmission more rapid.