bottom line, bull's-eye, centerpiece, core, crux, essence, gist, heart, keynote, meat, meat and potatoes, net, nub, nubbin, nucleus, pith, pivot, point, root, sum

course, direction, drift, tenorbody, content, substancehypothesis, proposition, purport, subject, theme, thesis

sum and substance, the long and short(or the long and the short)