Absorption term in the radiative transfer equation (RTE). Mathematical expression is \[\alpha_{\lambda} (\boldsymbol{x},t) L_{\mathit{\Omega},\lambda} (\boldsymbol{x}/t)\] with \(\alpha_{\lambda} (\boldsymbol{x},t)\) the space- and time-dependent linear napierian absorption coefficient and \(L_{\mathit{\Omega},\lambda} (\boldsymbol{x},t)\) the spectral radiance arriving at position \(x\) and time \(t\) from every possible angular directions \(\mathit{\Omega}\).