In Greek mythology, Mania ("insanity") was the personification of insanity.
In Roman mythology, originally borrowed from Etruscan mythology after the assimilation of their people, Mania was the goddess of the dead. She, along with Mantus, ruled the underworld. She was said to be the mother of ghosts and the undead, as well as the Lares and the Nanes.
Mania describes a condtion characterised by severely elevated mood. Associated with bipolar disorder, where episodes of mania alternate with episodes of depression. (Note: not all mania is bipolar disorder, other diseases can cause mania - however bipolar disorder is the 'classic' manic disease).
(Hypomania refers to a less severe variant of mania, where there is less loss of control.)
Although 'severely elevated mood' sounds pleasant, the actual experience of mania is usually unpleasant and frightening for the person involved.
Other manic symptoms include hypersexuality, religiosity, and hyperactivity.
Mania can also be experienced at the same time as depression, in so-called dysphoric mania.
This has caused speculation amongst doctors that mania and depression are two independent axes in a bipolar spectrum, rather than opposites.