Infrared (IR) radiation is a specific form of electromagnetic radiation, which is a form of energy that travels as a wave with a given frequency (number of waves in a certain time) and wavelength (distance between two wave-tops) that can be absorbed or emitted by matter.

When radiation has a high frequency, it will have a shorter wavelength (to "fit" more waves into a time period). Visible light, which constitutes the colors seen in day-to-day life, is a higher frequency form of electromagnetic radiation. The sun emits mostly visible light, while Earth emits and absorbs mostly infrared radiation, also commonly referred to as "thermal radiation."