General:
* IR (abbreviation)
* Longwave radiation (referring to the longer wavelengths of the electromagnetic spectrum)
* Heat radiation (emphasizing its thermal properties)
* Thermal radiation (similar to heat radiation)
Specific contexts:
* Near-infrared (for wavelengths closer to visible light)
* Far-infrared (for wavelengths further from visible light)
* Infrared light (emphasizing its nature as light)
* Infrared energy (emphasizing its energy aspect)
The best synonym will depend on the specific situation and what aspect of infrared you want to emphasize.