kernel, substance, core, center, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, pith, sum, nitty-gritty

the choicest or most essential or most vital part of some idea or experience; "the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"

• hypostasis — (metaphysics) essential nature or underlying reality
• quiddity, haecceity — the essence that makes something the kind of thing it is and makes it different from any other
• quintessence — the purest and most concentrated essence of something
• stuff — a critically important or characteristic component

• content, cognitive content, mental object — the sum or range of what has been perceived, discovered, or learned