Perhaps easier: if you can remember that "mono" means "one" (as in monophonic, monosyllable, monomania, monopoly, ...) then that should make it easy to remember

Perhaps easier: if you can remember that "mono" means "one" (as in monophonic, monosyllable, monomania, monopoly, ...) then that should make it easy to remember it's for (the analogue of) one-to-one functions -- same as injective. (But the OP is French and I don't know whether French mathematics terminology includes anything corresponding to English "one-to-one".)