Epsom salts

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural Hydrated magnesium sulfate, MgSO₄·7H₂O, used as a cathartic and as an agent to reduce inflammation.

from the GNU version of the Collaborative International Dictionary of English.

• (Med.) Sulphate of magnesia having cathartic qualities; -- originally prepared by boiling down the mineral waters at Epsom, England, -- whence the name; afterwards prepared from sea water; but now from certain minerals, as from siliceous hydrate of magnesia.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Common name for magnesium sulfate. Used as a laxative and in the preparation of tofu as a coagulator. Formerly commonly used as a medicated soaking solution.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun hydrated magnesium sulfate that is taken orally to treat heartburn and constipation and injected to prevent seizures
• noun (used with a singular noun) hydrated magnesium sulfate used as a laxative