Active ingredient means any substance (or group of structurally similar substances if specified by the Agency) that will prevent, destroy, repel or mitigate any pest, or that functions as a plant regulator, desiccant, or defoliant within the meaning of FIFRA sec. 2, except as provided in 174.3 of this chapter.
40 CFR § 152.3
Terms used in this part have the same meaning as in the Act. In addition, the following terms have the meanings set forth in this section.