From ZooTerms (Dictionary of Invertebrate Zoology)
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fold (noun; Anglo-Saxon folde, a fold): 1. A doubling or folding.

2. (Brachiopoda) A major external elevation of the valve surface, convex in transverse profile and radial from the umbo.

3. a. (Mollusca: Bivalvia) A broad undulation in the shell surface which is directed radially or comarginally. b. (Mollusca: Gastropoda) The spirally wound ridge on the interior of a shell wall

See also: columellar fold, parietal fold.