Entry 1 sense Webster, 1913 Infrared Radiation /ĭnˌ-frẽr-ĕd' rādˌ-ē-āsh'-ən/ · In·fra·red· Ra·di·a·tion · IPA 01 n. electromagnetic radiation having a wavelength of between 700 nanometers and 1 millimeter. 1. electromagnetic radiation having a wavelength of between 700 nanometers and 1 millimeter.(Physics)