pectin

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

• noun Any of a group of water-soluble colloidal carbohydrates of high molecular weight found in ripe fruits, such as apples, plums, and grapefruit, and used to jell various foods, drugs, and cosmetics.

from The Century Dictionary.

• noun A substance obtained from pectose by the action of heat, ferments, or an acid, and also formed in the ripening of fruits. It is soluble in water, and its solution on evaporating yields a fine jelly.

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

• noun (Chem.) One of a series of carbohydrates, commonly called vegetable jelly, found very widely distributed in the vegetable kingdom, especially in ripe fleshy fruits, as apples, cranberries, etc. It is extracted as variously colored, translucent substances, which are soluble in hot water but become viscous on cooling. It is commonly used in making fruit jelllies.

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

• noun biochemistry A polysaccharide extracted from the cell walls of plants, especially of fruits; under acidic conditions it forms a gel. It is often used in processed foods, especially jellies and jams where it causes thickening (setting).

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

• noun any of various water-soluble colloidal carbohydrates that occur in ripe fruit and vegetables; used in making fruit jellies and jams