Having a frequency in the electromagnetic spectrum in the range just below that of red light. Objects radiate infrared in proportion to their temperature. Infrared radiation is traditionally divided into four somewhat arbitrary categories based on its wavelength. See the table. Acronym: ir. Having or employing wavelengths longer than light but shorter than radio waves; lying outside the visible spectrum at its red end; infra-red

1. Electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves; SYN. infrared light, infrared radiation, infrared emission.
2. The infrared part of the electromagnetic spectrum; electromagnetic wave frequencies below the visible range; SYN. infrared frequency.
An area in the electromagnetic spectrum extending beyond red light from 760 nanometers to 1000 microns (106 nm). It is the form of radiation used for making non-contact temperature measurements.