hydra

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of several small freshwater polyps of the genus Hydra and related genera, having a naked cylindrical body and an oral opening surrounded by tentacles.

Century Dictionary and Cyclopedia

1. n. In Greek myth, a monstrous serpent or dragon of the lake or marsh of Lerna in Argolis, represented as having nine heads, each of which, being cut off, was immediately succeeded by two new ones unless the wound was cauterized. The destruction of this monster was one of the twelve labors of Hercules.
2. n. Hence—2. Figuratively, multifarious evil; evil or misfortune arising from many sources and not easily to be surmounted.
3. n. [capitalized] An ancient southern constellation, representing a sea-serpent. It is of Babylonian origin, like most of the ancient constellations. It is bounded by the ancient constellations Canis Minor, Argo, Centaurus, Virgo, Corvus, Crater, Leo, and Cancer, and by the modern constellations Sextans and Monoceros (which separates it from Canis Major). It contains one star of the second magnitude, and about four hundred stars visible to the naked eye.
4. n. In zoology: A venomous sea-snake; any one of the Hydrophidæ of the Indian ocean.
5. n. In Hydrozoa: [capitalized] A genus of fresh-water polyps of very simple structure, typical of the family Hydridæ. Among the species are H. viridis, H. fusca, and H. vulgaris. The body has the form of a cylindrical tube, composed of two fundamental layers, the ectoderm and endoderm, the former containing in one variety green granules identical with the chlorophyl of plants. The base is disk-shaped, and by it the animal can attach itself to any body, being capable of shifting its position. The mouth is surrounded by a circle of extremely contractile tentacles, by which the animal obtains its food, and which are richly endowed with the urticating organs or thread-cells common in the order. The mouth opens immediately into the stomach, and there are no internal organs of any kind, and no anal orifice. Each part of a hydra divided into almost any number of fragments will develop into a fresh independent polypite. Reproduction is effected by gemmation as well as by means of ova and sperm-cells. The genus is said to have been first described by Trembley in 1774, but it is attributed by Agassiz to Linnæus (1756), and the animal was described by A. van Leeuwenhoek in 1703. See cut under Hydrozoa.
6. n. An individual or a species of the genus Hydra.
7. n. The sexual bud or medusa of any hydroid hydrozoan: so called from its resemblance to a species of the genus Hydra.
8. n. A form of self-registering thermometer having a compound head or bulb to contain the spirits, with the object of increasing the surface exposed to the air, and thus making the instrument work with great rapidity.

Wiktionary

1. n. Any of several small freshwater polyps of the genus Hydra and related genera, having a naked cylindrical body and an oral opening surrounded by tentacles.
2. n. A complex, multifarious problem or situation that cannot be solved easily and rapidly.

GNU Webster's 1913

1. n. (Class. Myth.) A serpent or monster in the lake or marsh of Lerna, in the Peloponnesus, represented as having many heads, one of which, when cut off, was immediately succeeded by two others, unless the wound was cauterized. It was slain by Hercules. Hence, a terrible monster.
2. n. Hence: A multifarious evil, or an evil having many sources; not to be overcome by a single effort.
3. n. (Zoöl.) Any small fresh-water hydroid of the genus Hydra, usually found attached to sticks, stones, etc., by a basal sucker.
4. n. (Astron.) A southern constellation of great length lying southerly from Cancer, Leo, and Virgo.

WordNet 3.0

1. n. a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer
2. n. small tubular solitary freshwater hydrozoan polyp
3. n. trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality
4. n. (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones