Etymonline credits William James with creating the term ‘multiverse’ in 1895, but there is an earlier mention of the term by William Denovan, scientist and author of popular science books on the correspondence page of ‘Scientific American’ magazine, Nov 22, 1873, some 22 years earlier.
‘I must record my conviction that Science never can advance to a generalization of all the forces of Nature until it recognizes the fact that the substratum of mechanical power, appertaining to every unit, is as infinite and eternal as space and time in the will of God — that the Great Mechanic presides over a universe, and not merely a cohering multiverse.’
It’s possible that the term multiverse was in use even before then.
Great! Managed to find it, thank you.