isinglass

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

• noun A transparent, almost pure gelatin prepared from the swim bladder of the sturgeon and certain other fishes and used as an adhesive and a clarifying agent.
• noun Mica in thin, transparent sheets.

from The Century Dictionary.

• noun The purest commercial form of gelatin, a substance of firm texture and whitish color, prepared from the sounds or air-bladders of certain fresh-water fishes.
• noun Mica: so called from its resemblance to some forms of the gelatin.

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

• noun A semitransparent, whitish, and very pure form of gelatin, chiefly prepared from the sounds or air bladders of various species of sturgeons (as the Acipenser huso) found in the rivers of Western Russia. It used for making jellies, as a clarifier, etc. Cheaper forms of gelatin are not unfrequently so called. Called also fish glue.
• noun (Min.) A popular name for mica, especially when in thin sheets.

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

• noun A form of gelatine obtained from the air bladder of the sturgeon and certain other fish, used as an adhesive and as a clarifying agent for wine and beer.
• noun A thin, transparent sheet of mica.

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

• noun any of various minerals consisting of hydrous silicates of aluminum or potassium etc. that crystallize in forms that allow perfect cleavage into very thin leaves; used as dielectrics because of their resistance to electricity