n. A cryptogamic plant of the class Fungi: applied in a general sense to almost any of the larger, conspicuous fungi, such as toadstools, puffballs, Hydnei, etc., but more particularly to the agaricoid fungi and especially to the edible forms.n. n. An upstart; one who rises rapidly from a low condition in life.n. A small mushroom-shaped protuberance that sometimes forms on the end of the negative carbon in arc-lamps.Of or pertaining to mushrooms; made of mushrooms: as, mushroom sauce.Resembling mushrooms in rapidity of growth and in unsubstantiality; ephemeral; upstart: as, mushroom aristocracy.To elevate suddenly in position or rank.Having the form or shape of a mushroom or toadstool; formed like the segment of a sphere; said of valves, anchors, etc.To spread out at the top in a form resembling that of a mushroom; curve over at the top; curve down from the top like a mushroom.