Full Idea
According to the traditional account of individual essence, each thing has only one individual essence and it includes all the characteristics that the thing has necessarily.
Gist of Idea
A traditional individual essence includes all of a thing's necessary characteristics
Source
Roderick Chisholm (Person and Object [1976], 1.4)
Book Reference
Chisholm,Roderick: 'Person and Object' [Open Court 1976], p.29
A Reaction
Chisholm is steeped in medieval theology, but I don't think this is quite what Aristotle meant. Everyone nowadays has to exclude the 'trivial' necessary properties, for a start. But why? I'm contemplating things which survive the loss of their essence.