glacial acetic acid

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

• noun Acetic acid that is at least 99.8 percent pure. It is normally in a clear crystalline form near room temperature.

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

• noun organic chemistry Acetic acid that is at least 99.8% pure and tends to crystallize on cold days.