One @M03980@ of photons. Although widely used, it is not an IUPAC sanctioned unit. It is sometimes defined as the energy of one @M03980@ of photons. This use is discouraged.
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2238 [Terms] [Paper]
See also:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2186 [Terms] [Paper]