One that kills specific undesirable plants, sparing other desirable plants; this is done through different types of toxic action or by the manner in which the material is used (its formulation, dosage, timing, placement, etc.).
A chemical that kills plants in certain classes but not others. The contrasting term is "non-selective" herbicide, a chemical that kills all or nearly all plants.
Many herbicides are selective in what they control. That's why we can take broadleaf weeds out of your lawn or, control grasses in your flowerbeds. See also Non Selective Herbicide above.