the.com/button mushroom

means The common, mild-flavored white mushroom (Agaricus bisporus) picked while young and small, before it's grown into a cremini or fully matured into a portobello.

from 'Button' here is a shape word, dating to a time when English speakers used it loosely for any small, round, knob-like thing — buttons on coats, buttons of a plant, buttons of a mushroom cap. The mushroom itself, Agaricus bisporus, got its scientific name from Agaricus (an old genus name for gilled mushrooms, traced back to a Latin word some sources connect to a region once called Agaria) plus bisporus, Latin-Greek for 'two-spored,' describing how its gills carry spores two at a time instead of the usual four. The 'button' stage is simply the mushroom's earliest harvest point — so young its cap hasn't yet opened past that tight, round knob shape.