Definition: (noun) The choicest or most essential or most vital part of some idea or experience.
Synonyms: essence, heart and soul, inwardness, gist, pith, substance, meat, nub, kernel, sum, core, marrow, center.
Usage: While I will devote some time to Thomas Edison’s personal life, his many inventions will compose the nitty-gritty of my presentation.
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Source: The Free Dictionary