野草，亦作杂草，一般指在庭园、草坪或农地等土地上并非刻意栽种的植物。这些植物并不只限于草本植物。更多时候，野草专指有侵害性的植物，特别是那些不需栽种而能够自行大量繁殖的植物。植物被视为野草与否，属主观判断，有些野草在特定地区或族群不受欢迎，但在其他地区或族群有可能因为可入药、食用、观赏等用途，或不产生威胁，而不被视为野草。

野草不受欢迎的原因各有不同。植物可以因不美观而被视为野草。很多时候，野草会消耗泥土中的营养，又或遮闭阳光，阻碍较受欢迎的植物或农作物生长。部分野草带有毒性，会降低周边植物的产量或经济价值，或会容纳及散布对农作物有害的致病原，因而被刻意清除。

A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place". Examples commonly are plants unwanted in human-controlled settings, such as farm fields, gardens, lawns, and parks. Taxonomically, the term "weed" has no botanical significance, because a plant that is a weed in one context is not a weed when growing in a situation where it is in fact wanted, and where one species of plant is a valuable crop plant, another species in the same genus might be a serious weed, such as a wild bramble growing among cultivated loganberries. Many plants that people widely regard as weeds also are intentionally grown in gardens and other cultivated settings. The term also is applied to any plant that grows or reproduces aggressively, or is invasive outside its native habitat. More broadly "weed" occasionally is applied pejoratively to species outside the plant kingdom, species that can survive in diverse environments and reproduce quickly; in this sense it has even been applied to humans.