Definition of marsh orchis in US English:

marsh orchis

noun

rare
  • (Originally) a supposedly single orchid species of the genus Orchis found in marshes and damp meadows in Europe, later recognized as comprising two or more species and subspecies; (in later use) any of the segregates into which the ‘marsh orchis’ (as originally understood in Britain) was divided.

Origin

Early 19th century; earliest use found in John Vaughan Thompson (1779–1847), surgeon and naturalist.