Real Dictionary

infrared
Infrared equipment detects infrared radiation.
Infrared radiation is similar to light but has a longer wavelength, so we cannot see it without special equipment.

infrared in American English
designating or of those invisible rays just beyond the red end of the visible spectrum: their waves are longer than those of the spectrum colors but shorter than radio waves, and have a penetrating heating effect: used in cooking, photography, etc.
adjective: noting or pertaining to the infrared or its component rays
noun: the part of the invisible spectrum that is contiguous to the red end of the visible spectrum and that comprises electromagnetic radiation of wavelengths from 800 nm to 1 mm

infrared in British English
adjective: of, relating to, using, or consisting of radiation lying within the infrared
noun: the part of the electromagnetic spectrum with a longer wavelength than light but a shorter wavelength than radio waves; radiation with wavelength between 0.8 micrometres and 1 millimetre