rock salt

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Rock containing halite as its main constituent.
2. n. Coarsely ground or granulated common salt.

Century Dictionary and Cyclopedia

1. n. Salt existing in nature in the solid form, as distinguished from salt in solution, either in sea-water or in salt springs or lakes. Rock-salt made into prisms and lenses is invaluable in the study of the distribution of heat in the spectrum of the sun or other spectra, and in similar investigations, since it is very highly diathermanous even to the rays of long wave-length, which are largely absorbed by glass. See sælt.

Wiktionary

1. n. mineralogy The mineral halite
2. n. Coarsely ground common salt

GNU Webster's 1913

1. n. (Min.) cloride of sodium (common salt) occuring in rocklike masses in mines; mineral salt; salt dug from the earth. In the United States this name is sometimes given to salt in large crystals, formed by evaporation from sea water in large basins or cavities.

WordNet 3.0

1. n. naturally occurring crystalline sodium chloride