pistia vs water plant

pistia and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
pistia	Yes	No	No	No
water plant	Yes	No	No	No

As nouns, water plant is a hypernym of pistia; that is, water plant is a word with a broader meaning than pistia:

pistia: pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes
water plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth

Other hypernyms of pistia include aquatic plant, hydrophyte, hydrophytic plant.

pistia (noun)	water plant (noun)
pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes	a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth