All of those terms could be used interchangeably if you don't want to get too technical. I sort of like the term soul used to denote a single incarnation and rely on the term essence to represent the collective self, or repository that contains all previous incarnations, including the kitchen sink, I suspect. But I don't think it matters that much.
And you could make a similar argument with the term, fragment. I've heard it used to represent both essence and the individual incarnation. Essence is indeed a fragment of the Tao and the soul in our body is a fragment of essence. It's all the same source energy, though, so how you choose to assign these terms is a matter of personal preference.