From The Collaborative International Dictionary of English v.0.48 [gcide]:
1. To convert (the skin of an animal) into leather, as by usual process of steeping it in an infusion of oak or some other bark, whereby it is impregnated with tannin, or tannic acid (which exists in several species of bark), and is thus rendered firm, durable, and in some degree impervious to water.
Note: The essential result in tanning is due to the fact that the tannins form, with gelatins and albuminoids, a series of insoluble compounds which constitute leather. Similar results may be produced by the use of other reagents in place of tannin, as alum, and some acids or chlorides, which are employed in certain processes of tanning.
From WordNet (r) 3.0 (2006) [wn]:
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