ɪɾ ɪz ɐ mˈɛʒɚɹ ʌv hˌaʊ fˈoʊtɑːnz ɪn ɐ lˈaɪt bˈiːm ɑːɹ ɹᵻmˈuːvd ænd ɹᵻplˈeɪst baɪ nˈuː fˈoʊtɑːnz baɪ ðə mətˈɪɹiəl ɪt pˈæsᵻz θɹˈuː.
It is a measure of how photons in a light beam are removed and replaced by new photons by the material it passes through.