The term weed is used both in technical and general senses. In general senses relevant here it traditionally has been defined in part as: "A herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation... Applied to a shrub or tree, especially to a large tree, on account of its abundance in a district... An unprofitable, troublesome, or noxious growth." In general therefore, a weed is a plant that is considered by the user of the term to be a nuisance. The word commonly is applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More vaguely, "weed" is applied to any plants that grow and reproduce aggressively and invasively.

In technical modern usage the concept of a weed is that of a particular class of ecological relationship: a plant in an undesired place or under undesired circumstances. In weed ecology some authorities speak of the relationship between the three Ps: Plant, Place, Perception. These have been very variously defined, but the weed traits listed by H.G. Baker are widely cited.

Weeds may be unwanted for a number of reasons. The most important one is that they interfere with food and fiber production in agriculture, wherein they must be controlled in order to prevent lost or diminished crop yields. The next most important reason is that they interfere with other cosmetic, decorative, or recreational goals, such as in lawns, landscape architecture, playing fields, and golf courses. In all of these forms of horticulture, functional and cosmetic, weeds interfere by