rallentando
adverb or adjective
ral·len·tan·do ˌrä-lən-ˈtän-(ˌ)dō 
: ritardando
Word History
Etymology
Italian, literally, slowing down, verbal of rallentare to slow down again, from re- + allentare to slow down, from Late Latin, from Latin al- ad- + lentus slow, pliant — more at lithe
First Known Use
1786, in the meaning defined above