Before I start answering this question, let me clarify two essential points.
What is an outline in an essay?
An outline is a basic structure of your essay, your roadmap, guidance. It is a plan of what you are about to write. It depends on you, whether you want it super detailed or general.
What is outlining in essay writing?
Well, outlining is everything. If your essay were a house, the outline would be a ground, basic and pillars.
Without an outline (or a plan) an essay falls apart, thoughts are mixed, there is no order and no logic in the essay. Nobody would get the point. Nobody would know why’d you even started writing.
So now, let’s think of the essential parts of the essay outline.
I’ve already mentioned, that it depends on whether you want your plan detailed or general. I usually use the outline, that is something in between general and super detailed.
This is how it looks for me in general. Each essay would have own details and formulations in the outline, but the general structure would look like this.