Correct spelling for the English word "Decidence" is [dɪsˈa͡ɪdəns], [dɪsˈaɪdəns], [d_ɪ_s_ˈaɪ_d_ə_n_s] (IPA phonetic alphabet).
The act of falling off; downfall.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.