Noun | 1. | Pisonia - genus of often thorny tropical trees and shrubs and some vines; mainly America caryophylloid dicot genus - genus of relatively early dicotyledonous plants including mostly flowers Allioniaceae, family Allioniaceae, family Nyctaginaceae, four-o'clock family, Nyctaginaceae - a family of flowering plants of the order Caryophyllales Pisonia aculeata, cockspur - small spiny West Indian tree |