Also called a selective herbicide.
A weed killer that is more poisonous to some types of plants than others. Note that it selective weed killers are first weed killer, i.e. they kill all plants, and then the selective part is just how susceptible different plants are that particular chemical. Or to put it other way if you are not careful to make sure you follow the instructions accurately you will either kill off nothing or everything including the plants you what to keep.