Infrared is electromagnetic radiation which ranges from 700 nm to 1,000 nm (1 millimeter) wavelengths, and from 430 THz to 300 GHz frequencies.
Infrared is almost completely invisible to the human eye, and is felt as heat in some frequencies.
Photobiomodulation (light therapy) uses near and far infrared energy.
Night vision goggles and fiber optics use near infrared (NIR) frequencies (214-400 THz).
Long distance communication uses a subset of the short wavelength infrared frequencies (100-214 THz).
Heat-seeking missles target engine heat emitting a portion of the mid wavelength (intermediate) infrared (37-100 THz) frequencies.
Thermal imaging cameras capture the body temperature difference between room temperature and human warmth using the long wavelength infrared (20-37 THz) frequencies.
Some scientific probes and military weapons use the far infrared (0.3-20 THz) frequencies.