WordNet sense 1 (any of various squash plants grown for their elongated fruit with smooth dark green skin and whitish flesh):

marrow squash, vegetable marrow

WordNet sense 2 (very tender and very nutritious tissue from marrowbones):

bone marrow

WordNet sense 3 (the choicest or most essential or most vital part of some idea or experience):

center, core, essence, gist, heart, kernel, meat, nitty-gritty, nub, pith, substance, sum, inwardness, heart and soul