substance (generic term)
core (generic term)
center (generic term)
centre (generic term)
essence (generic term)
gist (generic term)
heart (generic term)
heart and soul (generic term)
inwardness (generic term)
marrow (generic term)
meat (generic term)
nub (generic term)
pith (generic term)
sum (generic term)
nitty-gritty (generic term)