hart UK /hɑːt/US /hɑɹt/ noun4 •A male deer, especially the male of the red deer after his fifth year.With milke-white Hartes vpon an Iuorie sled, Thou shalt be drawen amidst the frosen Pooles, And scale the ysie mountaines lofty tops: Which with thy beautie will be soone resolu’d. — 1587 •In the RISC-V instruction set architecture, a hardware thread.It is required by the RISC-V specification that at least one hart be assigned an ID of 0. •The meat from this animal.We are to have hart for dinner on Jack’s birthday; you call it deer in Grenada. — 1891 •(no longer used) The stag of any deer species. formsHARTs · harts