hibernaculum

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

• noun A protective case, covering, or structure, such as a plant bud, in which an organism remains dormant for the winter.
• noun The shelter of a hibernating animal.

from The Century Dictionary.

• noun Same as hibernacle, 1.
• noun In dot., any part of a plant which protects an embryonic organ during the winter, as a bud or bulb. Also hibernacle.
• noun In zoology: One of the winter buds of a polyzoan; an arrested and encysted polyzoön-bud capable of surviving the winter and germinating in the following spring.
• noun The false opercule or pseudoperculum of a snail.

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

• noun (Bot.) A winter bud, in which the rudimentary foliage or flower, as of most trees and shrubs in the temperate zone, is protected by closely overlapping scales.
• noun (Zoöl.) A little case in which certain insects pass the winter.
• noun Winter home or abiding place.

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

• noun zoology The place where a hibernating animal shelters for the winter
• noun botany A bud, case, or protective covering that a plant uses to survive the challenging environmental conditions during a dormancy period.