Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mītĭgātĭo, ōnis, f. [mitigo], a soothing, mitigating, mitigation (rare but class.), Cic. de Or. 3, 30, 118: multis mitigationibus lenire, Auct. Her. 4, 37, 49: quae non indiget mitigationis, id. ib. 4, 37, 50: si est lingua curationis, est et mitigationis, Vulg. Sir. 36, 25.