pigweed, name for several weedy plants, particularly the common pigweed or lamb's-quarters of the family Chenopodiaceae (goosefoot family), the rough pigweed, or green amaranth, of the related family Amaranthaceae (amaranth family), and the winged pigweed, a tumbleweed. Pigweeds are classified in the division Magnoliophyta, class Magnoliopsida, order Caryophyllales.