Definition of hydra

• n. - 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.
• n. - Hence: A multifarious evil, or an evil having many sources; not to be overcome by a single effort.
• n. - Any small fresh-water hydroid of the genus Hydra, usually found attached to sticks, stones, etc., by a basal sucker.
• n. - A southern constellation of great length lying southerly from Cancer, Leo, and Virgo.