A telegram was a type of written communication transmitted over long distances by a telegraph machine. Such technology was typically used in or prior to the 20th century.
Telegrams were part of The Adventures of Captain Proton holoprogram created by Tom Paris. In the program, they were received as a strip of paper with the message printed on it for the intended recipient. Each sentence was concluded with the word "stop".
While in the program, he received a telegram in his rocket ship from the Earth government. It stated that aliens from the fifth dimension were at war with Doctor Chaotica and his Army of Evil. (VOY: "Bride of Chaotica!")