Definitions of infrared:

• noun:   the infrared part of the electromagnetic spectrum; electromagnetic wave frequencies below the visible range

  Example: "They could sense radiation in the infrared"

• noun:   electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves
• adjective:   having or employing wavelengths longer than light but shorter than radio waves; lying outside the visible spectrum at its red end

  Example: "Infrared radiation"