To send to all rather than to one. Broadcasting reaches everyone and answers nobody.
Compound of broad (Old English brad) and cast (Old Norse kasta, to throw); the word first named scattering seed by hand across a wide field, and was borrowed for radio in the 1920s on exactly that image — sowing a signal wide rather than aiming it.
To send to everyone rather than to one — a broadcast reaches every listener and answers none of them directly.