It has long been assumed that weeds, in the sense of rapidly-evolving plants taking advantage of human-disturbed environments, evolved in response to the Neolithic agricultural revolution approximately 12,000 years ago. However, researchers have found evidence of "proto-weeds" behaving in similar ways at Ohalo II, a 23,000-year-old archeological site in Israel.[30]

Van Dale beschrijft het als: Ongewenste plant(en) tussen gekweekte gewassen. Maar in principe kan elke plantensoort dus als onkruid worden beschouwd. Er is geen objectieve of biologische definitie van onkruid.