The afterword technically only mentions there being "two" meanings of tensei, but there's actually a few that could be applicable (though my guess is the last two):

天性 "nature; natural constitution; innate disposition"
転成 "transformation"
転生 "reincarnation"
点睛 "adding eyes and other finishing touches to an animal painting"

Levander said:

The afterword technically only mentions there being "two" meanings of tensei, but there's actually a few that could be applicable (though my guess is the last two):

天性 "nature; natural constitution; innate disposition"
転成 "transformation"
転生 "reincarnation"
点睛 "adding eyes and other finishing touches to an animal painting"

Probably it’s from 画竜点睛. So it would be 点睛