Any chemical used to destroy or inhibit plant growth, especially of weeds or other undesirable vegetation. There are well over a hundred chemicals in common usage as herbicides. Many of these are available in several formulations or under several trade names. The variety of materials are conveniently classified according to the properties of the active ingredient as either selective or nonselective. Selective herbicides are those that kill some members of a plant population with little or no injury to others. Nonselective herbicides are those that kill all vegetation to which they are applied. Further subclassification is by method of application, such as preemergence (soil-applied before plant emergence) or postemergence (applied to plant foliage). Additional terminology sometimes applied to describe the mobility of post-emergence herbicides in the treated plant is contact (nonmobile) or translocated (mobile—that is, killing plants by systemic action).

A rapidly expanding use for nonselective herbicides is the destruction of vegetation before seeding in the practice of reduced tillage or no tillage. Some are also used to kill annual grasses in preparation for seeding perennial grasses in pastures. Additional uses are in fire prevention, elimination of highway hazards, destruction of plants that are hosts for insects and plant diseases, and killing of poisonous or allergen-bearing plants.

Preemergence or postemergence application methods derive naturally from the properties of the herbicidal chemical. The distinction between pre- and postemergence is not always clear-cut. For example, atrazine can exert its herbicidal action either following root absorption from a preemergence application or after leaf absorption from a postemergence treatment.