Grinding mill means a machine used for the wet or dry fine crushing of any nonmetallic mineral. Grinding mills include, but are not limited to, the following types: Hammer, roller, rod, pebble and ball, and fluid energy. The grinding mill includes the air conveying system, air separator, or air classifier, where such systems are used.
40 CFR § 60.671
All terms used in this subpart, but not specifically defined in this section, shall have the meaning given them in the Act and in subpart A of this part.