Radio waves are an invisible form of electromagnetic radiation that varies in wavelength from around a millimeter to 100,000 km, making it one of the widest ranges in the electromagnetic spectrum. "Radio" is a catch-all term describing all forms of electromagnetic radiation with a wavelength longer than a millimeter and a frequency above 300 GHz. Frequency refers to how long the measured time is between the "crest" and "trough" of a wave arriving at the source. For visible light, the frequency is in the 450-750 terahertz range, meaning in a single second of waves, 450-750 crests and troughs pass through a detector.

Radio is most famously utilized to send audio, images, and text in the form of signals. Guglielmo Marconi and Nikola Tesla are both credited with being early pioneers in the area of radio. Radio's long wavelength allows it to circumvent obstacles and travel long distances, unlike visible light and other spectra of higher frequencies. Marconi patented the first wireless telegraphy system in 1896.

Radio waves with a wavelength of less than around 10 meters are absorbed by the atmosphere. Otherwise, they bounce back and forth between the ionosphere and the ground, making radio ideal for transmitting over the horizon. The lowest frequencies are used for communication with submarines, due to their low energy (for stealth) and high penetrating power. Consider lower frequencies to have more "bass," meaning they penetrate further, especially through thick media such as water.

Radio waves were first predicted in 1865 by James Clerk Maxwell, who came up with the equations for electromagnetism, later known as Maxwell's equations. The existence of these waves was demonstrated experimentally only 12 years later, in 1887, when Heinrich Hertz generated radio waves in his lab. Within a few decades, radio waves were used widely for transmitting information.

The longest radio waves typically encountered by humans are generated using the entire Earth as an antennae. These waves have a wavelength similar to the diameter of Earth.