Board Thread:General Discussion/@comment-27464043-20151230192612/@comment-28278649-20180216171431

Like I said before, it would make no sense for the Soul Stone to have been in Wakanda. I was hoping it was the collective power of the Asgardians, but I'm thinking it won't be revealed until Infinity War as a sort of climax to the buildup of the MCU, much like how Infinity War itself is.

As the name suggests, it is possible it's someone's actual life force - someone of significant power (kinda have a meta-theory that it'll be the Watcher Informant).