(n.) A stone serving the same purpose as a milepost.

bench mark bookmark cairn cardinal point catstone chief thing climax core cornerstone crisis critical point crux essence essential essential matter fundamental gist gravamen great point heart high point important thing issue kernel keystone landmark lighthouse lightship main point main thing mark marker material point meat menhir milepost monument nub occasion pharos pith pivot platform real issue salient point seamark sine qua non substance substantive point the bottom line the point tower turning point watchtower