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, allotments gardens, lawns, and parks.
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.
In the same way, volunteer crops (plants) are regarded as weeds in a subsequent crop.
Many plants that people widely regard as weeds also are intentionally grown in gardens and other cultivated settings, in which case they are sometimes called beneficial weeds or wildflowers.
The term weed also is applied to any plant that grows or reproduces aggressively, or is invasive outside its native habitat.
Not being able to get to my allotment for four months during the first Corona Virus wave, my plots are covered with weeds, and it occurred to me that I could not identify many of them and wanted too and to find out the best ways of dealing with them and should they be Burnt, taken from the plot, safe to compost directly of did they need to be dried or drowned before adding to the Dalek Army?