Simultaneous (coherent) absorption of two photons (either same or different wavelength), the energy of excitation being the sum of the energies of the two photons. Also called two-photon excitation.
This term is sometimes also used for a two-step absorption.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 308 (https://doi.org/10.1351/pac200779030293)