tuber

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

• noun A swollen, fleshy, usually underground outgrowth of the stem or rhizome of a plant, such as the potato, bearing buds from which new plant shoots arise.
• noun A similar outgrowth of a plant root.
• noun A rounded projection or swelling; a tubercle.

from The Century Dictionary.

• noun In botany, a subterranean body, usually of an oblong or rounded form, consisting morphologically of a stolon-like branch of a rhizome, much thickened, commonly at the end, and beset with “eyes,” which are properly modified axillary buds.
• noun A genus of subterranean discomycetous fungi, the truffles, having the peridium warty or tubercled, without definite base, the asci ovoid or globose, and one- to three- or (rarely) four-spored. About 50 species are known. T. æstivum is the common truffle. See truffle (with cut).
• noun In pathol., anat., and zoology, some rounded swelling part; a tuberosity; a tubercle; a knot or swelling which is not the result of disease: used chiefly as a Latin word (with Latin plural tubera).

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

• noun A fleshy, rounded stem or root, usually containing starchy matter, as the potato or arrowroot; a thickened root-stock. See Illust. of tuberous.
• noun A genus of fungi. See truffle.
• noun (Anat.) A tuberosity; a tubercle.

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

• noun A fleshy, thickened underground stem of a plant, usually containing stored starch, as for example a potato or arrowroot.
• noun horticulture A thickened "root-stock".

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

• noun a fleshy underground stem or root serving for reproductive and food storage
• noun type genus of the Tuberaceae: fungi whose fruiting bodies are typically truffles