kernel

(noun) the inner and usually edible part of a seed or grain or nut or fruit stone. Synonyms: meat. Example: "Black walnut kernels are difficult to get out of the shell."

(noun) a single whole grain of a cereal. Example: "A kernel of corn."

(noun) the choicest or most essential or most vital part of some idea or experience. Synonyms: substance, core, center, centre, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, pith, sum, nitty-gritty. Examples: "The gist of the prosecutor's argument." "The heart and soul of the republican party." "The nub of the story."